read_table fails with MultiIndex input and delim_whitespace=True
Related #6889

Example:

``` python
In [4]: text = """                      A       B       C       D        E
one two three   four
a   b   10.0032 5    -0.5109 -2.3358 -0.4645  0.05076  0.3640
a   q   20      4     0.4473  1.4152  0.2834  1.00661  0.1744
x   q   30      3    -0.6662 -0.5243 -0.3580  0.89145  2.5838"""

In [5]: pd.read_table(StringIO(text), delim_whitespace=True)
---------------------------------------------------------------------------
CParserError                              Traceback (most recent call last)
. . .
CParserError: Error tokenizing data. C error: Expected 6 fields in line 3, saw 9
```

This (partially) works if `delim_whitespace=True` is replaced with `sep='\s+', engine='python'` (although columns A-D are lost):

``` python
In [6]: pd.read_table(StringIO(text), sep='\s+', engine='python')
Out[6]: 
                           E
one two three   four        
a   b   10.0032 5     0.3640
    q   20.0000 4     0.1744
x   q   30.0000 3     2.5838

[3 rows x 1 columns]
```
