BUG: Fix isnan with postgres

[llllllllll]

sqlite does not have a NaN value so I don't test against it.

[cpcloud]

wacky.

from the postgresql docs: http://www.postgresql.org/docs/current/static/datatype-numeric.html

Note: In most implementations of the "not-a-number" concept, NaN is not considered equal to any other numeric value (including NaN). In order to allow numeric values to be sorted and used in tree-based indexes, PostgreSQL treats NaN values as equal, and greater than all non-NaN values.

[cpcloud]

So apparently there is something bigger than infinity:

pcloud=# select float8 'NaN' > float8 'Infinity';
 ?column?
----------
 t
(1 row)

[cpcloud]

@llllllllll thanks!