fix: preserve existing query string parameters when adding enum encoding option

[alexander-fenster]

Background: to enable numeric enum responses for the REST transport, the client must add $alt=json%3Benum-encoding=int to the query string.

Apparently, the logic that added the parameter did not preserve the existing query string content, so the requests that relied on query strings failed. We don't have that many requests relying on query strings, but the one failing in Firestore (just one from the whole test suite) was one of them.