ENH: Use case-insensitive checks for 'inf' in parser.

Instead of just 'inf' or '-inf', can test for 'Inf', '-Inf', 'INF', etc.
Uses strcasecmp under the hood.

(also, small fix to assert_almost_equal to make string comparisons
clearer)
Merge pull request #4220 from jtratner/expand-inf-comparisons

ENH: Treat 'Inf' as infinity in text parser