… for 1xx, 204 and 304 responses (playframework#6261)

As described in RFC7230 section-3.3.2[1], sending a Content-Length
header field is not allowed in any response with a status code of
1xx (Informational) or 204 (No Content). This requirement was not forced
in Play, which means a user can intentionally set a Content-Length header
field like you find from this test code[2].

With regard to a Content-Length header field in a 304 response, we don't
allow users to do it currently even a server May send it as mentioned
in RFC7230 section-3.3.2.

Additionally, as described in RFC7230 section-3.3[3], 1xx responses don't include
a message body. With this commit, a message body isn't sent to meet the requirement.

Then this fix makes sure that Play never sends a `Content-Length` header field
and message-body for 1xx, 204 and 304 responses.

The following is the partial citation from RFC7230 section-3.3.2 and 3.3.
```
A server MUST NOT send a Content-Length header field in any response with
a status code of 1xx (Informational) or 204 (No Content). A server MUST NOT send
a Content-Length header field in any 2xx(Successful) response to a CONNECT request
(Section 4.3.6 of [RFC7231]).
```

```
A server MAY send a Content-Length header field in a 304 (Not Modified) response to
a conditional GET request (Section 4.1 of [RFC7232]); a server MUST NOT send
Content-Length in such a response unless its field-value equals the decimal number of
octets that would have been sent in the payload body of a 200 (OK) response to the same request.
```

```
All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
responses do not include a message body.  All other responses do
include a message body, although the body might be of zero length.
```

[1] https://tools.ietf.org/html/rfc7230#section-3.3.2
[2] https://github.com/playframework/playframework/blob/2.5.4/framework/src/play-integration-test/src/test/scala/play/it/http/ScalaResultsHandlingSpec.scala#L327
[3] https://tools.ietf.org/html/rfc7230#section-3.3

…gProtocols}

As we can't add methods to trait if we want to maintain binary-compatibility,
I avoided adding these vals to Results.scala.