Prior to this commit, the `ShallowEtagHeaderFilter` could participate in
the response and set its ETag/Content-Length headers, even for HEAD
requests. Since the response body is empty, the filter implementation
would set a `"Content-Length: 0"`.
The RFC states that responses to HEAD requests should exhibit identical
response headers to GET (with the possible exception of payload related
headers such as Content-Length.
With this commit, `ShallowEtagHeaderFilter` now ignores HEAD requests
since the proper values may be set already for payload related headers
by the handler. The filter has no way to generate a proper ETag value
nor calculate the content length without the actual body.
Issue: SPR-15261
(cherry picked from commit b732251)
45b27a85f4