controllers/helpers/pagination: Remove generic parameter from gather() fn

[Turbo87]

Ideally we'd only pass in a Uri, but since we're using the RequestLog we still need to pass in the full Parts struct.

Previously we were compiling two variants of this function, one for Parts and one for Request, although only our test code was still using the Request variant. This PR changes the tests to use Parts too and then removes the obsolete generic parameter from the gather() fn.