Remove mutation that was adding extra query params

[danhaller]

POST endpoints with parameter substitution e.g. ~/foo/{bar} are not working after v7. The user is told that query string parameters cannot be used in combination with payloads, even if the parameter is substituted into the url.

I tracked this down to unintentional mutation in the RequestBuilder.BuildOAuthHeader method, which was adding the substitution parameters back in to the query string parameters after they had been removed.

I've fixed the mutation and renamed a couple of variables to make it a little clearer.

[danhaller]

Uh I've just realised I change the wrong test, will fix in a sec

[danhaller]

ok, done