BUG: expm of integer matrices

(cherry picked from commit a8d2f843c5b047f04e025783dab438721293476d)
TST: strengthen the test for expm of integer matrices

(cherry picked from commit 9c579f1d6e34d1484352d608af0642164ead364b)
Merge pull request #4162 from cournape/backport_expm_integer_fix

Backport expm integer fix