BUG: Fix pandas test for version update

Iris pandas unit test currently fails due to a dtype mismatch between
what pandas Timestamp class can handle with v.11 compared to v.12  This
commit converts the input argument which instantiation this class
to an integer, which is commonly accepted by both versions of pandas.
Merge pull request #650 from cpelley/pandas_version_fix

BUG: Fix pandas test for version update