Added support for RFC2231 (Parameter Value Continuations)

Some params for header values are split and urldecoded (see [RFC2231](https://tools.ietf.org/html/rfc2231#section-3)). This update makes sure that such multi line values are concatenated into "normal" mime encoded word values. RFC2231 additionally modifies mime-encoded word structure by adding *lang property to the encoding part ("UTF-8" -> "UTF-8*EN"). This updated removes the language part from the encoding.