Canonicalise the request according to the site

Each site can have its own rules in the form of which query parameters are
significant and the order in which they should be compared.

This brings us closer to behaviour parity with redirector.
Throw away parameters that are not significant

We should ignore query string parameters that are not listed as significant for
the site in question.
Add a regression test for blank query_params

We should treat a blank query_params value as an empty list, which (thankfully)
optic14n is already doing for us.
Merge pull request #24 from alphagov/canonicalise_query_according_to_site

Canonicalise the request according to the site