Ignore the ``No Content-Type`` error raised by pygit2
    
    We recently starting running into this error for remote PR opened from a
    github git repo. Maybe something change in github that is now triggering
    this.
    Anyway, with this commit, we will now be ignoring that specific error
    and keep on going as usual.
    It will at least allow accessing pull-requests that have already been
    open and which are basically still valid.
    
    Fixes https://pagure.io/pagure/issue/1863