Decode payload body based on message body Transfer-Encoding.

[samoconnor]

"The Transfer-Encoding header field lists the transfer coding names corresponding to the sequence of transfer codings that have been (or will be) applied to the payload body in order to form the message body."
https://tools.ietf.org/html/rfc7230#section-3.3.1

The Messages.payload function calls Messages.decode, but that function does not yet do anything:

HTTP.jl/src/Messages.jl

Lines 386 to 389 in b602c4e

	function payload(m::Message)::Vector{UInt8}
	enc = lowercase(first(split(header(m, "Transfer-Encoding"), ", ")))
	return enc in ["", "identity", "chunked"] ? m.body : decode(m, enc)
	end

HTTP.jl/src/Messages.jl

Lines 395 to 401 in b602c4e

	function decode(m::Message, encoding::String)::Vector{UInt8}
	if encoding == "gzip"
	# Use https://github.com/bicycle1885/TranscodingStreams.jl ?
	end
	@warn "Decoding of HTTP Transfer-Encoding is not implemented yet!"
	return m.body
	end

[quinnj]

Dup of #256