Docserver: Fix _patch URLs by fixing optarg handling in instance_servlet.py.

R=jyasskin@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/27260008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228935 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228935