Fix file seek error in boto3 - closes #382

[nikolas]

This is the solution from @charlesthk here:
#382 (comment)

[0011001011]

Any plan on making it pass test then merge ? I happened to have this problem and it is becoming more and more common.

I got the reference from this thread :
https://www.bountysource.com/issues/48279727-s3boto3storage-raises-valueerror-i-o-operation-on-closed-file

[nikolas]

@0011001011 it looks like it's failing on some encoding errors, like this:

    def write(self, s):
        file = self._file
>       rv = file.write(s)
E       TypeError: a bytes-like object is required, not 'str'

I will address this unless someone else gets to it first.

[jschneier]

Merged #754 to fix this instead, thanks for your contribution.