Fix closing of new unsaved project

1. Create a new project.
2. Import an asset.
3. Close project without saving.

You will notice that the project doesn't close and we get a traceback:

Traceback (most recent call last):
  File "/home/harish/Pitivi/pitivi/pitivi/editorperspective.py", line 402, in __close_project_cb
    self.app.project_manager.closeRunningProject()
  File "/home/harish/Pitivi/pitivi/pitivi/project.py", line 493, in closeRunningProject
    project.create_thumb()
  File "/home/harish/Pitivi/pitivi/pitivi/project.py", line 852, in create_thumb
    thumb_path = self.get_thumb_path(self.uri, ORIGINAL_THUMB_DIR)
  File "/home/harish/Pitivi/pitivi/pitivi/project.py", line 803, in get_thumb_path
    thumb_hash = md5(quote_uri(uri).encode()).hexdigest()
  File "/home/harish/Pitivi/pitivi/pitivi/utils/misc.py", line 182, in quote_uri
    raw_path = unquote(parts.path)
  File "/usr/lib/python3.5/urllib/parse.py", line 537, in unquote
    if '%' not in string:
TypeError: a bytes-like object is required, not 'str'