return 400 instead of 444 when spam detected

because 444 is a nginx-specific return code, with our current
configuration, returning 444 doesn't work as intended (
ie headers and a response body are still returned).
After this change, no response body will be returned.
set the skip_slimmer header, rather than returning empty body

this is equivalent, but setting the header is a bit more explicit
Merge pull request #58 from alphagov/edge-case-improvements

Edge case improvements