Parse Content-Disposition headers more relaxed.

Before, we got an unhandled ValueError if there were multiple spaces in a
filename.

Instead of ignoring such a header altogether, we now add relaxed=True which
still tries to get the filename in this situation (and with a semicolon at the
line end).

Because the parser is also more relaxed in that second case, we mark a test as
expected failure.

qutebrowser/test/utils/test_misc_content_disposition.py

@@ -251,6 +251,8 @@ class AttachmentTests(AttachmentTestCase):
         """
         self._check_ignored('attachment; filename=foo,bar.html')
 
+    # With relaxed=True we accept that
+    @unittest.expectedFailure
     def test_attwithasciifilenamenqs(self):
         """'attachment', specifying a filename of foo.html.
 
@@ -881,6 +883,16 @@ class RFC2047EncodingTests(AttachmentTestCase):
             '=?ISO-8859-1?Q?foo-=E4.html?=')
 
 
+class OurTests(AttachmentTestCase):
+
+    """Our own tests, not based on http://greenbytes.de/tech/tc2231/"""
+
+    def test_att_double_space(self):
+        """'attachment' with double space in the filename."""
+        self._check_filename('attachment; filename="foo  bar.html"',
+                             'foo bar.html')
+
+
 def setUpModule():
     """Mock out logging in utils."""
     utils.log = Mock()

qutebrowser/utils/misc.py

@@ -322,7 +322,7 @@ def parse_content_disposition(reply):
         # os.path.basename later.
         try:
             content_disposition = rfc6266.parse_headers(
-                bytes(reply.rawHeader('Content-Disposition')))
+                bytes(reply.rawHeader('Content-Disposition')), relaxed=True)
             filename = content_disposition.filename_unsafe
         except UnicodeDecodeError as e:
             log.misc.warning("Error while getting filename: {}: {}".format(