Call .json etc. 'request_format' instead of 'format'

Because this was conflicting with 'format' as a search term:

`/government/advanced_search?page=1&per_page=1&format=some_document_format`

Rummager was misinterpreting this as asking for data to be returned
in a mime type called 'some_document_format'.
Merge pull request #203 from alphagov/fix-advanced-search-with-format-key

Call .json etc. 'request_format' instead of 'format'