strip trailing and leading linear whitespace in headers

strip trailing and leading linear whitespace in headers

RFC 2616, section 4.2:
> The field-content does not include any leading or trailing LWS:
> linear white space occurring before the first non-whitespace
> character of the field-value or after the last non-whitespace
> character of the field-value. Such leading or trailing LWS MAY be
> removed without changing the semantics of the field value. Any LWS
> that occurs between field-content MAY be replaced with a single SP
> before interpreting the field value or forwarding the message
> downstream.