Support identity transfer-encoding in httpc

<blockquote>The default (identity) encoding; the use of no transformation whatsoever.</blockquote>

<blockquote>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).</blockquote>

@aahoughton