Strip `Path` suffix in `NewRedirectHandler`

Because both of these arguments might contain query strings, saying
they're Paths is wrong.
Explicitly test that query strings are preserved

This adds a test to ensure query strings are preserved for targets. As
part of this rename the original query string test to make it clear that
it's aimed at the source of the redirect, not the target.
Merge pull request #78 from alphagov/support-query-strings-in-redirects

Validate support for query strings in redirects