[Bug #8182] XMLRPC request fails with "Wrong size. Was 31564, should be 1501"

* lib/net/http/response.rb: transparently deflates the response body,
  removes the "content-encoding" response header, but does not adjust
  the "content-length" header accordingly.
  So, pass the context to the Inflater so that we count the
  uncompressed data for every chunk inflated, and then on finish we
  set the right Content-Length.
  [Bug #8182]

* test/net/http/test_httpresponse.rb: enhance testcase to check
  the right Content-Length