BUG: GH3611 fix again, float na_values were not stringified correctly

     now, 999.0 (a float) will have: ['999','999.0'] added for matching
BUG: add na_fvalues to parsers to enable matching of float values w/o relying on string matching

     for an exact match
Merge pull request #3841 from jreback/GH3611_2

BUG: GH3611 fix again, float na_values were not stringified correctly