Fix deflate encoded responses

RFC 2616 (HTTP/1.1) says in the "3.5 Content Codings" [1] section about deflate:

    The "zlib" format defined in RFC 1950 [31] in combination with the "deflate" compression mechanism described in RFC 1951 [29].

The zlib package [2] from Go's standard library imlpements RFC 1950 and it also uses[3] the flate package internally, which implements RFC 1951 [4]

[1] https://tools.ietf.org/html/rfc2616#section-3.5
[2] https://golang.org/pkg/compress/zlib/#pkg-overview
[3] https://golang.org/src/compress/zlib/writer.go#L15
[4] https://golang.org/pkg/compress/flate/#pkg-overview