Stop zero from sneaking into mbox calculation
Stop zero from sneaking into mbox calculation

Changed the mbox calculation to use same structure as zbox, while preserving the intent of setting mbox to zeros in case of no m values. 

In the previous implementation the list of m values was instantiated with a zero, presumably to set an mbox of zeros in case of no m values, but this would eg force the mbox to be 0-30 in a case where the true range was 15-30. Unless there is something I don't know about m values, this should not be the case.
jzmiller1/pyshp-1,GeospatialPython/pyshp