httpreplay: ignore X-Goog-Api-Client header on replay
httpreplay: ignore X-Goog-Api-Client header on replay

The header is different for different versions of Go, which causes replay
to fail when done with a version that differs from the one used to record.

Make this part of httpreplay's list of ignored headers, since it will
come up for everyone.

Tested by running against Go 1.8 and 1.9.

Change-Id: I2da144050f83eebcbd2fdbf4e6cae2235813daea
Reviewed-on: https://code-review.googlesource.com/29930
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go