Fix check for file-like objects

The `is_fileobject()` function in utils.py would break when the respones
was a wrapped `HTTPResponse`'s `raw` attribute. This just adds the
`IOError` exception type to the `is_fileobject()` function so that the
response can be streamed normally.

Fixes #805
Merge pull request #806 from benoitc/805-bug-is-fileobj

Fix check for file-like objects