Don't crash on Content-Disposition with wrong enc.

When declaring UTF-8 but encoding RFC2231 ISO-8859-1, rfc2259 raises an
UnicodeDecodeError.

Test case: http://greenbytes.de/tech/tc2231/#attwithfn2231iso-bad

qutebrowser/browser/downloads.py

@@ -344,8 +344,12 @@ class DownloadManager(QObject):
         if reply.hasRawHeader('Content-Disposition'):
             # We use the unsafe variant of the filename as we sanitize it via
             # os.path.basename later.
-            filename = rfc6266.parse_headers(
-                bytes(reply.rawHeader('Content-Disposition'))).filename_unsafe
+            try:
+                content_disposition = rfc6266.parse_headers(
+                    bytes(reply.rawHeader('Content-Disposition')))
+                filename = content_disposition.filename_unsafe
+            except UnicodeDecodeError:
+                filename = None
         else:
             filename = None
         # Then try to get filename from url