- Execute content filters through execute_content_filter().

- Execute content filters through execute_content_filter().
- Add prepare_for_filtering() so filter functions don't have to
  care about de-chunking and decompression. As a side effect this enables
  decompression for gif_deanimate_response() and jpeg_inspect_response().
- Change remove_chunked_transfer_coding()'s return type to jb_err.
  Some clowns feel like chunking empty responses in which case
  (size == 0) is valid but previously would be interpreted as error.