Stabilize response headers for subresource integrity

[twifkak]

For #13 to be effective, the header-integrity value for a given URL should remain relatively stable. The worker should eliminate frequently changing response headers that don't affect the semantics of the SXG (e.g. Date). Resources for researching which headers to eliminate:

https://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Response_fields
https://datatracker.ietf.org/doc/html/rfc7230 and friends