BUG: log(exposure) is attached to GLM

Closes #2019 
Since we are no longer attaching it here (just checking the length),
calling np.asarray wouldn't do anything.

On Mon, Sep 29, 2014 at 8:55 AM, Josef Perktold <notifications@github.com>
wrote:

> In statsmodels/genmod/generalized_linear_model.py:
>
> > @@ -230,17 +230,12 @@ def _check_inputs(self, family, offset, exposure, endog):
> >              raise ValueError("exposure can only be used with the log link function")
> >
> >          if offset is not None:
> > -            offset = np.asarray(offset)
>
> why did you remove the asarray?
>
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> <https://github.com/statsmodels/statsmodels/pull/2020/files#r18151423>.
>