Add test for decoded_content bug

Furl::Response::decoded_content returns undef if user specifies
'Accept-Encoding' for data compression. Because content is already
inflated, so HTTP::Message::decoded_content failed and return undef.
Fix decoded_content bug
remove header, not substitute string

I mistake data structure of Furl::Headers
Add comment why removes 'Content-Encoding' field
Merge pull request #59 from syohex/fix_decoded_content

Fix decoded_content