Is your feature request related to a problem? Please describe.
In Storage, we use the Accept-Encoding header to allow users to download the zipped version of their data, see ReadCompressed. We cannot support this in JSON because the header is disallowed.

Describe the solution you'd like
Allow the header, at least for storage media downloads. This requires changes to the generated code.

Describe alternatives you've considered
Allow the header in general. This can be done in gensupport. (edit: this actually would be my preferred solution, but it may impact other libraries - linked PR is for the first option)