Convert URL to proxy correctly.

Before, user/password was set to an empty string even when not set in
the URL, causing Qt to reread this information all the time and
call proxyAuthenticationRequired even when the entered info was correct.

qutebrowser/config/_conftypes.py

@@ -677,7 +677,14 @@ class Proxy(BaseType):
         url = QUrl(value)
         if (url.isValid() and not url.isEmpty() and
                 url.scheme() in self.PROXY_TYPES):
-            return
+            if url.userName() and url.password():
+                pass
+            elif not url.userName() and not url.password():
+                pass
+            else:
+                raise ValidationError(value, "must either have user and "
+                                             "password or none of both")
+        else:
             raise ValidationError(value, "must be a proxy URL (http://... or "
                                          "socks://...) or system/none!")
 
@@ -696,8 +703,14 @@ class Proxy(BaseType):
             return QNetworkProxy(QNetworkProxy.NoProxy)
         url = QUrl(value)
         typ = self.PROXY_TYPES[url.scheme()]
-        return QNetworkProxy(typ, url.host(), url.port(), url.userName(),
-                             url.password())
+        proxy = QNetworkProxy(typ, url.host())
+        if url.port() != -1:
+            proxy.setPort(url.port())
+        if url.userName():
+            proxy.setUser(url.userName())
+        if url.password():
+            proxy.setPassword(url.password())
+        return proxy
 
 
 class SearchEngineName(BaseType):