Merge branch 'mk/http-backend-content-length'

The earlier attempt barfed when given a CONTENT_LENGTH that is set to an empty string. RFC 3875 is fairly clear that in this case we should not read any message body, but we've been reading through to the EOF in previous versions (which did not even pay attention to the environment variable), so keep that behaviour for now in this late update. * mk/http-backend-content-length: http-backend: allow empty CONTENT_LENGTH