BUG: Fix for to_excel +/- infinity

Previously, a negative sign was being prepended to positive infinite
values and was absent for negative infinity. (GH6812)

TST: assert_almost_equals() checks +/- infinity

The existing ``test_inf_roundtrip`` should have caught this bug, but
the underlying ``assert_almost_equal`` was not checking that the two
given values were both positive or negative.