HttpStatus: Implement header() method

HttpStatus: Implement header() method

Standardise this. Currently MediaWiki does this in many different ways.

> WebResponse->header( "HTTP/1.0 $code $message" );
> WebResponse->header( "HTTP/1.1 $code $message" );
> WebResponse->header( "HTTP/1.x $code $message" );
> WebResponse->header( "HTTP/1.1 $code " . HttpStatus::getMessage( $code ) );
> WebResponse->header( "HTTP/1.1 $code $message", true, $code );
> header( "Status: $code $message", true, $code );

Change-Id: I1551e89808a1a67310549ec8d9fcb387754c9169