Quality values not considered when parsing Accept-Encoding request header

[damb]

Quality values are not taken into consideration when parsing the Accept-Encoding request header field. The encoding chosen depends on the ordering in the ContentCoding enum:

aiohttp/aiohttp/web_response.py

Lines 51 to 58 in cd5c48a

	class ContentCoding(enum.Enum):
	# The content codings that we have support for.
	#
	# Additional registered codings are listed at:
	# https://www.iana.org/assignments/http-parameters/http-parameters.xhtml#content-coding
	deflate = 'deflate'
	gzip = 'gzip'
	identity = 'identity'

and

aiohttp/aiohttp/web_response.py

Lines 333 to 338 in cd5c48a

	accept_encoding = request.headers.get(
	hdrs.ACCEPT_ENCODING, '').lower()
	for coding in ContentCoding:
	if coding.value in accept_encoding:
	await self._do_start_compression(coding)
	return

However, RFC 7231, section 5.4.3 states

4.  If multiple content-codings are acceptable, then the acceptable
       content-coding with the highest non-zero qvalue is preferred.

Though, within a note RFC 7231 also mentions that

Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings.  This means that qvalues might
      not work and are not permitted with x-gzip or x-compress.

Actually I'm not sure if this issue is a bug or rather a feature request. Though, I think that it would be a reasonable improvement. Would you accept a PR?