Don't pass include_children param to the Content API

The include_children parameter was removed from the Content API in #98 
and it's no longer used. This removes the parameter from the with_tag 
helper and changes the tests that make requests with it.
Fix a typo in a Content API test helper

Not quite sure how this got missed :(
Make the Content API adapter test use test helpers

We should be using the test helpers to stub out the requests here, so 
that we can be sure that they work as expected.
Don't rely on the query string order for with_tag helper

The Content API adapter test helper which stubs out the with_tag.json 
endpoint breaks when the tag type comes earlier in the alphabet than 
'sort', because the query string gets rearranged when the request is 
made.

This rewrites the Webmock stub calls to use a hash instead of a string, 
so that the parameters are matched regardless of the order they take in
the URI.
Add a new Content API test helper for sort order

Whilst there's already a test helper that stubs out the with_tag 
endpoint with a 'sort' parameter, this test helper stubs out just the
URI with the 'sort' parameter, and allows for a custom sort order to 
be provided.
Don't use CGI.escape in Webmock's hash stub format

This breaks the tests in Frontend because it no longer matches slashes 
in URL requests. I think it's fine to not escape these URLs when we're 
providing them to Webmock as a hash.
Rename test helper method
Merge pull request #113 from alphagov/tidy-up-test-helpers

Improve the Content API test helpers (again)