ApplyParameters should respect existing querystring parameters

Because without that respect, the `page=N` parameter is dropped and operations that require paging will continue to fetch the first page until your API limits are exceeded...

Open questions:

 - ~~Do you want an integration test for this scenario? (this was found during testing the milestones client) or could we just mock the response to see that we respect paging?~~ nevermind
 - ~~The implementation itself is rather naive (the existing tests pass) and I'm not overly attached to the code itself - should we test it more thoroughly while we're in here?~~ reviewed