TST: failing windows parser test
This test is failing on all versions of windows (but not linux).

related to #7582, #7591 
cc @amras1
cc @mcwitt

not sure what is failing this

```
FAIL: test_concat_invalid_first_argument (pandas.tools.tests.test_merge.TestConcatenate)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win-amd64-2.7\pandas\tools\tests\test_merge.py", line 2107, in test_concat_invalid_first_argument
    assert_frame_equal(result,expected)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win-amd64-2.7\pandas\util\testing.py", line 641, in assert_frame_equal
    check_exact=check_exact)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win-amd64-2.7\pandas\util\testing.py", line 588, in assert_series_equal
    assert_almost_equal(left.values, right.values, check_less_precise)
  File "testing.pyx", line 58, in pandas._testing.assert_almost_equal (pandas\src\testing.c:2465)
  File "testing.pyx", line 93, in pandas._testing.assert_almost_equal (pandas\src\testing.c:1793)
  File "testing.pyx", line 69, in pandas._testing.assert_almost_equal (pandas\src\testing.c:1489)
AssertionError: nan != 'bar'

```

can you guys take a look...thxs

Here's some debug output

```
C:\Users\Jeff Reback\Documents\GitHub\pandas>c:\python27-64\Scripts\nosetests.exe build\lib.win-amd64-2.7\pandas\tools\tests\test_merge.py --pdb --pdb-failure
................> c:\users\jeff reback\documents\github\pandas\testing.pyx(69)pandas._testing.assert_almost_equal (pandas\src\testing.c:1489)()
(Pdb) u
> c:\users\jeff reback\documents\github\pandas\testing.pyx(93)pandas._testing.assert_almost_equal (pandas\src\testing.c:1793)()
(Pdb) u
> c:\users\jeff reback\documents\github\pandas\testing.pyx(58)pandas._testing.assert_almost_equal (pandas\src\testing.c:2465)()
(Pdb) u
> c:\users\jeff reback\documents\github\pandas\build\lib.win-amd64-2.7\pandas\util\testing.py(588)assert_series_equal()
-> assert_almost_equal(left.values, right.values, check_less_precise)
(Pdb) u
> c:\users\jeff reback\documents\github\pandas\build\lib.win-amd64-2.7\pandas\util\testing.py(641)assert_frame_equal()
-> check_exact=check_exact)
(Pdb) u
> c:\users\jeff reback\documents\github\pandas\build\lib.win-amd64-2.7\pandas\tools\tests\test_merge.py(2107)test_concat_invalid_first_argument()
-> assert_frame_equal(result,expected)
(Pdb) l
2102    """
2103
2104            reader = read_csv(StringIO(data), chunksize=1)
2105            result = concat(reader, ignore_index=True)
2106            expected = read_csv(StringIO(data))
2107 ->         assert_frame_equal(result,expected)
2108
2109    class TestOrderedMerge(tm.TestCase):
2110
2111        def setUp(self):
2112            self.left = DataFrame({'key': ['a', 'c', 'e'],
(Pdb) p result
  index     A   B   C   D
0   foo     2   3   4   5
1   NaN  ?¶  n NaN NaN
2   baz    12  13  14  15
3   qux    12  13  14  15
4  foo2    12  13  14  15
5  bar2    12  13  14  15
(Pdb) p data
'index,A,B,C,D\nfoo,2,3,4,5\nbar,7,8,9,10\nbaz,12,13,14,15\nqux,12,13,14,15\nfoo2,12,13,14,15\nbar2,12,13,14,15\n'
(Pdb) p expected
  index   A   B   C   D
0   foo   2   3   4   5
1   bar   7   8   9  10
2   baz  12  13  14  15
3   qux  12  13  14  15
4  foo2  12  13  14  15
5  bar2  12  13  14  15
(Pdb) !reader = read_csv(StringIO(data),chunksize=1)
(Pdb) p list(reader)
[  index  A  B  C  D
0   foo  2  3  4  5,    index     A   B   C   D
0    NaN  ?¶  n NaN NaN,   index   A   B   C   D
0   baz  12  13  14  15,   index   A   B   C   D
0   qux  12  13  14  15,   index   A   B   C   D
0  foo2  12  13  14  15,   index   A   B   C   D
0  bar2  12  13  14  15]

```
