Support identity transfer-encoding in httpc

> The default (identity) encoding; the use of no transformation whatsoever.

> The Internet Assigned Numbers Authority (IANA) acts as a registry for
> transfer-coding value tokens. Initially, the registry contains the
> following tokens: "chunked" (section 3.6.1), "identity" (section 3.6.2),
> "gzip" (section 3.5), "compress" (section 3.5), and "deflate" (section 3.5).

Reported-by: Andrew Houghton