Fix exception when downloading links without name.

We also set a default name to prevent "is a directory" errors.

This is a regression introduced in 8f33fcfc52cf598d0aa11a347992c87010d3e37a.
Fixes #682.

CHANGELOG.asciidoc

@@ -73,6 +73,7 @@ Fixed
 - Various fixes for deprecated key bindings and auto-migrations.
 - Workaround for qutebrowser not starting when there are NUL-bytes in the history (because of a currently unknown bug)
 - Fixed handling of keybindings containing Ctrl/Meta on OS X.
+- Fixed crash when downloading an URL without filename (e.g. magnet links) via "Save as...".
 
 https://github.com/The-Compiler/qutebrowser/releases/tag/v0.2.1[v0.2.1]
 -----------------------------------------------------------------------

qutebrowser/browser/downloads.py

@@ -686,8 +686,11 @@ class DownloadManager(QAbstractListModel):
         if fileobj is not None or filename is not None:
             return self.fetch_request(request, page, fileobj, filename,
                                       auto_remove, suggested_fn)
-        encoding = sys.getfilesystemencoding()
-        suggested_fn = utils.force_encoding(suggested_fn, encoding)
+        if suggested_fn is None:
+            suggested_fn = 'qutebrowser-download'
+        else:
+            encoding = sys.getfilesystemencoding()
+            suggested_fn = utils.force_encoding(suggested_fn, encoding)
         q = self._prepare_question()
         q.default = _path_suggestion(suggested_fn)
         message_bridge = objreg.get('message-bridge', scope='window',