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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The mistake is here. `Div` operator has precedens over `minus`