FileSystem.create does not pass full Metadata to GCS

When I call *FileSystem.create(foo, mime_type="text/plain", compression_type=CompressionTypes.BZIP2)* I would expect the following metadata set on GCS Blob:
bq. content_type: text/plain
bq. content_encoding: application/bzip2

What we currently get is:
bq. content_type: application/x-bz2

Looking at the code it seems to be a problem with GcsIO interface, which does not seem to expose *content_type* and *content_encoding* properly.

Imported from Jira [BEAM-12714](https://issues.apache.org/jira/browse/BEAM-12714). Original Jira may contain additional context.
Reported by: abergmeier.