code: function _checkInvalidHeaderChar(val) {
  	val += '';
  	if (val.length < 1) {
  		return false;
  	}
  	var c = val.charCodeAt(0);
  	if (c <= 31 && c !== 9 || c > 255 || c === 127) {
  		return true;
  	}
  	if (val.length < 2) {
  		return false;
  	}
  	c = val.charCodeAt(1);
  	if (c <= 31 && c !== 9 || c > 255 || c === 127) {
  		return true;
  	}
  	if (val.length < 3) {
  		return false;
  	}
  	c = val.charCodeAt(2);
  	if (c <= 31 && c !== 9 || c > 255 || c === 127) {
  		return true;
  	}
  	for (var i = 3; i < val.length; ++i) {
  		c = val.charCodeAt(i);
  		if (c <= 31 && c !== 9 || c > 255 || c === 127) {
  			return true;
  		}
  	}
  	return false;
  }
docs: True if val contains an invalid field-vchar
  field-value    = *( field-content / obs-fold )
  field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
  field-vchar    = VCHAR / obs-text
  
  checkInvalidHeaderChar() is currently designed to be inlinable by v8,
  so take care when making changes to the implementation so that the source
  code size does not exceed v8's default max_inlined_source_size setting.
   /* istanbul ignore next