Intext 118: Fix message part splitting for long messages containing newlines

This pull request concerns INTEXT-118 in spring-integration-smpp. 

Long messages that contain newlines were not split into parts of correct size.  

The test in the pull request demonstrates that no matter if the message contains newlines or not the splitting results in correctly sized message parts.

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Ah, yes - I missed it because the code stille works when calling add on the list. :-/
This is my first pull request to one of the spring projects. How would you like me to proceed, do I fix the error in my branch, and create a new pull request - or what should I do?