fix (and test) for encoding submit_sm_resp messages with no message_id

Working with a provider who leaves off the message_id in error submit_sm_resp, I noticed that the PDUEncoder was throwing a ArrayIndexOutOfBounds exception when trying to append a \0 byte to the buffer.

Digging deeper, it seems this is actually acceptable behavior according to SMPP 3.4 (but not 3.3 or 5.0):

http://en.wikipedia.org/wiki/Short_Message_Peer-to-Peer#submit_sm_resp_incompatibility_between_SMPP_versions

It seems like some places in the code already account for this ambiguity, but the PDUEncoder did not.

There is one edge case this fix does not cover, but I do not think it is important. Consider a submit_sm_resp with a non-0 status, an omitted message_id, and the existence of optional parameters.  In this situation, I maintain that it's not always possible to disambiguate between when the message_id null-terminated-string ends and when the optional parameters begin; it seems like an oversight of the 3.4 spec.