fix decimal rescale values causing Decimal data frame

I ran into this bug using pydicom 0.9.7 and some Siemens Files
RescaleSlope and RescaleIntercept are returned as Decimal.

```HeaderDataError: data dtype "object" not recognized
> /home_local/bpinsard/.virtualenvs/processing_virtenv/lib/python2.6/site-packages/nibabel/analyze.py(578)set_data_dtype()
    577             raise HeaderDataError(
--> 578                 'data dtype "%s" not recognized' % datatype)
    579         dtype = self._data_type_codes.dtype[code]
```

dtype being object because Decimal