BUG: convert_objects(convert_numeric=True) fails with all strings
```
s=pd.Series(['a','a','a'])

s.convert_objects(convert_numeric=True)
Out[78]: 
0    a
1    a
2    a
dtype: object

s[0]=1.0

s
Out[80]: 
0    1
1    a
2    a
dtype: object

s.convert_objects(convert_numeric=True)
Out[81]: 
0     1
1   NaN
2   NaN
dtype: float64
```

Having a single number changes behavior.  Makes `convert_objects` unreliable when the data type must be numeric.
