Support HTTP HEAD request with query parameters.

Hi,

I suspect Rack::Test does not support HTTP HEAD request with query params.

I think the HEAD request should be able to handle query params as with the GET request.
But in the following code, HEAD is not treated same as GET.
https://github.com/futonaka/rack-test/blob/master/lib/rack/test.rb#L208

So I fixed this problem.
Would you check this pull request?

Thank you.

Futonaka