HTTP Compliance: delete Warnings that have warning-date different from Date.

HTTP Compliance: delete Warnings that have warning-date different from Date.

Added HttpReply::removeStaleWarnings() method that iterates over all Warning
headers and removes stale warning-values. If a reply has no valid Date header,
all warning-values with warning-date are removed. Also, we remove
warning-value if we failed to parse warning-date.

removeStaleWarnings() is called from processReplyHeader(), after reply headers
are parsed.

Co-Advisor test cases:
  test_case/rfc2616/date-accept-fmt-warn-asctime-rfc1123
  test_case/rfc2616/date-accept-fmt-warn-asctime-rfc850
  test_case/rfc2616/date-accept-fmt-warn-rfc1123-asctime
  test_case/rfc2616/date-accept-fmt-warn-rfc1123-rfc850
  test_case/rfc2616/date-accept-fmt-warn-rfc850-asctime
  test_case/rfc2616/date-accept-fmt-warn-rfc850-rfc1123