OANDA returning string for currency above 1000, creating error when reversing the currency

[STguerin]

OANDA is returning string when currency is above 1000, so when you try to reverse the currency ( 1/df ) it raise an error

Code example:

from pandas_datareader.oanda import get_oanda_currency_historical_rates

fx_rates = get_oanda_currency_historical_rates(start='1990-01-01', end=None, quote_currency='USD', base_currency='KRW')

Traceback (most recent call last):
  File "C:\Anaconda2\lib\site-packages\IPython\core\interactiveshell.py", line 2881, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-14c50a5808d3>", line 3, in <module>
    fx_rates = get_oanda_currency_historical_rates(start='1990-01-01', end=None, quote_currency='USD', base_currency='KRW')
  File "C:\Anaconda2\lib\site-packages\pandas_datareader\oanda.py", line 52, in get_oanda_currency_historical_rates
    df = 1 / df
  File "C:\Anaconda2\lib\site-packages\pandas\core\ops.py", line 1097, in f
    return self._combine_const(other, na_op)
  File "C:\Anaconda2\lib\site-packages\pandas\core\frame.py", line 3555, in _combine_const
    raise_on_error=raise_on_error)
  File "C:\Anaconda2\lib\site-packages\pandas\core\internals.py", line 2911, in eval
    return self.apply('eval', **kwargs)
  File "C:\Anaconda2\lib\site-packages\pandas\core\internals.py", line 2890, in apply
    applied = getattr(b, f)(**kwargs)
  File "C:\Anaconda2\lib\site-packages\pandas\core\internals.py", line 1139, in eval
    result = handle_error()
  File "C:\Anaconda2\lib\site-packages\pandas\core\internals.py", line 1123, in handle_error
    (repr(other), str(detail)))