Fixes a simple bug, and also an annoying warning message that has been printing for a while.
BUG: fix stats.distributions.geom.logpmf bug. Closes #1793.
Thanks to Jonathan Scholz.
TST: filter warning message from stats test.
Introduced by recent change in warning mechanism in special module.
Leaving the test unchanged, because it's a regression test.
Fitting with n=1 fixed would be better, but doesn't seem to be supported yet.
Looks good to me.