BUG: Check for NaN after data conversion to numeric
In an attempt to squash a Python parser bug in which weirdly-formed floats weren't being checked for `nan`, the bug was traced back to a bug in the `maybe_convert_numeric` function of `pandas/src/inference.pyx`.  Added tests for the bug in `test_lib.py` and adjusted the original `nan` tests in `na_values.py` to test all of the engines.
