Avoid multiple "Transfer-Encoding: chunked" header.

Multiple Transfer-Encoding is accepted on HTTP/1.1.
But, an error has occured on part of devices.
(I encounter the error on iPhone4S SoftBank 3G)

Additionally, response body that is returned by
Network.HTTP.Conduit is already decoded.
So, "Transfer-Encoding: chunked" header should be deleted.
Merge pull request #4 from melpon/delete-transfer-encoding

Avoid multiple "Transfer-Encoding: chunked" header.