INCLUDES FIRST PARTITION WHEN PRUNING


PROBLEM
-------

TO_DAYS()/TO_SECONDS() can return NULL for invalid dates which 
was stored in the first partition ,therefore the first partition 
was always included for the scan when range was specified.


FIX
---

The fix is a small optimization which we have included ,which will
prune the scanning of NULL/first partition if the dates specified 
in the range are valid and in the same year and month . TO_SECONDS()
function is not supported in 5.1 so removed it from the fix and test
scripts for mysql-5.1 version.