Consume all enumerators created by gzip filter

This fixes an issue with New Relic monitoring and GzipFilter.
The New Relic monitoring tracks all promises to completion,
and an unused empty enumerator caused problems with this.
Merge pull request #2363 from pvlugter/gzip-filter-enumerators-2.2

Consume all enumerators created by gzip filter (backport 2.2.x)