Escape commas in urls in whitehall mappings file

Some urls in whitehall mappings contain commas. Although a comma is a
valid character in a url path[1] the commas cause problems with the rest
of the munge/redirector processing pipeline.

[1] http://tools.ietf.org/html/rfc3986#section-3.3
Merge pull request #102 from alphagov/escape-commas-in-whitehall-mappings-urls

Escape commas in urls in whitehall mappings file