[Patch] ActiveResource::HttpMock overwrites Accept/Content-Type header

ActiveResource::HttpMock always overwrite the Accept/Content-Header parameter of the recorded response to "application/xml", making it impossible to mock two different responses to the same method/url but with different mime types.

ActiveResource::HttpMock.respond_to do |mock|
  mock.get "/people/1", {"Accept" => "application/xml"}, "XML"
  mock.get "/people/1", {"Accept" => "application/json"}, "Json"
end

@http.get("/people/1", {"Accept" => "application/json"}).body # > XML

This patch fixes that by 1) not overwriting the format header and 2) ignoring it on the response match on it's not set on the mock and add some basic tests to HttpMock, but I think the code can be improved.