REGTESTS: cache: Use rxresphdrs to only get headers for 304 responses

REGTESTS: cache: Use rxresphdrs to only get headers for 304 responses

304 responses contains "Content-length" or "Transfer-encoding"
headers. rxresp action expects to get a payload in this case, even if 304
reponses must not have any payload. A workaround was added to remove these
headers from the 304 responses. However, a better solution is to only get
the response headers from clients using rxresphdrs action.

If a payload is erroneously added in these reponses, the scripts will fail
the same way. So it is safe.