astropy: tabledump is broken when the output file is not specified

It seems that previously an HDU could access to its parent HDUList with a _HDUList__file attribute, and this was used to get the filename. I think the simplest solution would be to make the output file mandatory (datafile).

In [3]: fits.tabledump('astropy/io/fits/tests/data/tb.fits')
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-3-c48651aaf53f> in <module>()
----> 1 fits.tabledump('astropy/io/fits/tests/data/tb.fits')

~/dev/astropy/astropy/utils/decorators.py in wrapper(*args, **kwargs)
    485                         # one with the name of the new argument to the function
    486                         kwargs[new_name[i]] = value
--> 487             return function(*args, **kwargs)
    488 
    489         return wrapper

~/dev/astropy/astropy/io/fits/convenience.py in tabledump(filename, datafile, cdfile, hfile, ext, overwrite)
    888             # TODO: Really need to provide a better way to access the name of
    889             # any files underlying an HDU
--> 890             root, tail = os.path.splitext(f._HDUList__file.name)
    891             datafile = root + '_' + repr(ext) + '.txt'
    892 

AttributeError: 'HDUList' object has no attribute '_HDUList__file'