BUG/COMPAT: io.common.get_filepath_or_buffer should use pass encoding parameter

[TomAugspurger]

Ran into this when downloading a file from s3 with py3.

This line needs to pass the encoding parameter to the boto method .get_contents_as_string, else it returns a stream of bytes which trips up StringIO on python 3.

Simple fix. Will try to get to it tonight unless someone beats me to it.

[TomAugspurger]

@cpcloud I don't think this will mess anything up with into. It seems that you are passing in encoding=utf-8 (which is correct for my case), but I don't really understand how into handles kwargs.