Mail\Headers.php: Adjust regex to RFC 5322

According to RFC 5322 (http://tools.ietf.org/html/rfc5322#section-2.2), a field name may contain "printable US-ASCII characters (i.e., characters that have values between 33 and 126, inclusive), except colon".
Mail\Header/GenericHeader.php: Adjust regex to RFC 5322