read_csv does not apply dtype=str correctly to NA values
Hi,
It seems that read_csv() ignores the dtype=str when encountering NA values. For instance if the contents of file.csv is:

```
id,symbol
1,NA
NA,g2
3,g3
4,g4
```

Then `df=pandas.read_csv('file.csv', dtype={'id':float, 'symbol':str})` will return the following data frame:

```
   id symbol
0   1    NaN
1   NaN   g2
2   3     g3
3   4     g4
```

which means it casts NA value in the symbol column as a float. Is this a bug or part of the read_csv design? It would make sense that when dtype=str is specified all the values will be treated as string. Note that in the above example using 'keep_default_na=False' in read_csv will cause an error.
