Header key/value encoding validation

[hhurtado]

GO's implementation doesn't make any header key/value encoding validation. We need to add this validation.

[panchoh]

We have to discuss as well wether the cookie headers must be case sensitive or not.

[dumblob]

@panchoh unfortunately they kind of do (read threads https://github.com/vlang/v/pull/9462#issuecomment-808962169 and https://github.com/vlang/v/pull/9602#issuecomment-813105884 ).

We shall follow the basic robustness principle: be liberal on input and strictly conformant on output.