Bug 926048.
Filter checks path for user-defined forbidden characters, and for
user-defined maximum length.
Includes changes to reflect gholt's latest comments to Patch Set 4
Also includes a change to a unit-test, renames another unit-test,
and removes one superfluous unit-test.
Added section to the example proxy config
Fixed-up unit test pep8 warnings
Changed error response code to 400 (Bad Request)
Change-Id: Iace719d6a3d00fb3dda1b9d0bc185b8c4cbc00ca
cf1aa3c309