Make codecs lookup optional for HTTP methods that cannot contain requ…

[vellotis]

…est or response body.

By RFC7231, only some of the HTTP methods shall contain a request and response body. The mw.Codec method should not fail if the request or response doesn't require a body.

HTTP method	Request body	Response body
GET		Required
HEAD
POST	Required	Required
PUT	Required
DELETE	Optional	Optional
CONNECT		Required
OPTIONS		Required
TRACE
PATCH	Required	Required

Request body

The request body should be required

• IF
  HEADER Transfer-Encoding is defined and has other value than identity
• OR
  HEADER Content-Length value is greater than 0

which means that the Content-Type header should be present for them in the request headers.

By RFC7231 the condition of HEADER(Content-Length) < 0 or not a number or missing should be considered invalid and the body should be ignored. Only DELETE method may have Transfer-Encoding and Content-Length headers missing in which case the request doesn't have a body.

Response body

If the response body is required it means that the Accept header should be present in the request headers.