Use NoneString config type for settings with Qt defaults.

This fixes wikipedia looking ugly because setUserStylesheet gets called
with an empty string.

qutebrowser/browser/webpage.py

@@ -72,7 +72,7 @@ class BrowserPage(QWebPage):
     def userAgentForUrl(self, url):
         """Override QWebPage::userAgentForUrl to customize the user agent."""
         ua = config.get('network', 'user-agent')
-        if not ua:
+        if ua is None:
             return super().userAgentForUrl(url)
         else:
             return ua

qutebrowser/config/_conftypes.py

@@ -165,6 +165,7 @@ class String(BaseType):
         self.forbidden = forbidden
 
     def transform(self, value):
+        # FIXME that .lower() probably isn't always a good idea...
         return value.lower()
 
     def validate(self, value):
@@ -180,6 +181,16 @@ class String(BaseType):
                                   self.maxlen))
 
 
+class NoneString(String):
+
+    """String which returns None if it's empty."""
+
+    def transform(self, value):
+        if not value:
+            return None
+        return super().transform(value)
+
+
 class ShellCommand(String):
 
     """A shellcommand which is split via shlex.

qutebrowser/config/configdata.py

@@ -203,11 +203,11 @@ DATA = OrderedDict([
          "Value to send in the DNT header."),
 
         ('accept-language',
-         SettingValue(types.String(), 'en-US,en'),
+         SettingValue(types.NoneString(), 'en-US,en'),
          "Value to send in the accept-language header."),
 
         ('user-agent',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "User agent to send. Empty to send the default."),
 
         ('accept-cookies',
@@ -422,51 +422,51 @@ DATA = OrderedDict([
          "User stylesheet to set."),
 
         ('css-media-type',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Set the CSS media type."),
 
         ('default-encoding',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Default encoding to use for websites."),
 
         ('font-family-standard',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Font family for standard fonts."),
 
         ('font-family-fixed',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Font family for fixed fonts."),
 
         ('font-family-serif',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Font family for serif fonts."),
 
         ('font-family-sans-serif',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Font family for sans-serif fonts."),
 
         ('font-family-cursive',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Font family for cursive fonts."),
 
         ('font-family-fantasy',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "Font family for fantasy fonts."),
 
         ('font-size-minimum',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "The hard minimum font size."),
 
         ('font-size-minimum-logical',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "The minimum logical font size that is applied when zooming out."),
 
         ('font-size-default',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "The default font size for regular text."),
 
         ('font-size-default-fixed',
-         SettingValue(types.String(), ''),
+         SettingValue(types.NoneString(), ''),
          "The default font size for fixed-pitch text."),
 
         ('maximum-pages-in-cache',

qutebrowser/network/networkmanager.py

@@ -72,8 +72,10 @@ class NetworkManager(QNetworkAccessManager):
                 dnt = '0'.encode('ascii')
             req.setRawHeader('DNT'.encode('ascii'), dnt)
             req.setRawHeader('X-Do-Not-Track'.encode('ascii'), dnt)
-            req.setRawHeader('Accept-Language'.encode('ascii'),
-                             config.get('network', 'accept-language'))
+            accept_language = config.get('network', 'accept-language')
+            if accept_language is not None:
+                req.setRawHeader('Accept-Language'.encode('ascii'),
+                                 accept_language.encode('ascii'))
             reply = super().createRequest(op, req, outgoing_data)
             self._requests[id(reply)] = reply
             reply.destroyed.connect(lambda obj: self._requests.pop(id(obj)))