Remove Werkzeug bug workaround from flask/app.py

According to the note in the comment, you had to check to make sure that the
defaults were not an empty dictionary because of a bug in Werkzeug pre-0.7.

Since Flask officially requires 0.7 or greater, we can remove this little
workaround.
Merge pull request #811 from rpicard/remove_options_workaround

Remove Werkzeug bug workaround from flask/app.py