some exceptions fixed

[andrey-kazakov]

1. incompatible encoding exception (US-ASCII vs ASCII-8BIT) was raised in Smpp::Pdu::Base#initialize while reading some PDUs from SMPP bytestream. it's fixed just by String#force_encoding to ASCII-8BIT.
2. message_id was limited to be integer in Smpp::Tranciever#send_concat_mt. moreover, it's just ruined UDH structure if message_id was >255 and raised an exception if message_id was greater than unicode char range. it's fixed by using object_id of message_id (I don't think that internal ids must be integer anyway; I use hashes in my app, for example) and cutting it down to one octet ( & 0xFF).