Reject header fields with invalid whitespace characters.

[Lukasa]

Relevant: golang/go#16572.

We should forbid implementations from emitting header fields with invalid whitespace characters. Generally, that means surrounding whitespace of any kind, either leading or trailing, from both header field names and header field values.

[alexwlchan]

Is there anything left to do, or is this fixed by #281?

[Lukasa]

I think this is fixed by #281.