Add .json to fake match

We have feeds that contain the names of other feeds. I.e.: `channels/bbc_one/highlights` contains `channels`. That's why it's impossible at the moment to fake just the channels feed without faking the highlights or schedules feeds, since if you add the querystring `?_fake=channels@myfixture` both `channels/bbc_one/highlights` and `channels` will be a list of channels, which will fail for the highlights feed.

The solution is to add the format extension (that we are already adding in the actual request to iBL) to the string to match against the `fake` string. This way, if we only want to fake the `channels` feed, we just do
`_fake=channels.json@myfixture`