integration-test: update test for new sniffing logic
integration-test: update test for new sniffing logic

Commit aabc760 changed the Content-Type behaviour so that auto content
sniffing is disabled if the user specified a content-type on the
provided object.

Change-Id: I7a89ea287502c462855efc6a9133c31d73c718e9
Reviewed-on: https://code-review.googlesource.com/5531
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

google/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client,googleapis/google-api-go-client