Refactor GZIP encoding check

[ndegwamartin]

Description of what I changed

When determining whether to apply the Gzip header our condition checks for an exact match in the header value passed for Accept-Encoding. This is limiting since various clients by default supply multiple comma separated values e.g. Accept-Encoding: gzip, deflate, br and the server responds according to configuration.

A better approach would be to use the String.contains method to check for the occurrence of the GZip literal.

E2E test

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[ndegwamartin]

@bashir2 a few things to note with the PR.

1. The changes doc/design.md documentation are because of applying spotless, let me know if you need me to revert this.
2. Should we care about case sensitivity when performing the check. There may be a performance implication. So something like acceptEncodingValue.toLowerCase(Locale.ENGLISH).contains(GZIP_ENCODING_VALUE);

[bashir2]

Thanks for the fixes @ndegwamartin ; just some minor suggestions.