For the record, it looks like there were a few distinct but related problems here:
* Passing any StringIO object as the body is not supported (see <https://docs.python.org/2/library/httplib.html#httplib.HTTPConnection.request>), because no StringIO class implements the fileno() method.
* Passing a Python 2 natively implemented StringIO.StringIO instance triggers the same AttributeError exception as in Issue 15267, incompatibility with TemporaryFile objects.
* Even if HTTPConnection.request() did support calculating Content-Length for StringIO objects, it would still be wrong for chunk-encoded bodies. I think the lower-level endheaders() or send() methods should be used instead. Or see Issue 12319, about adding chunked encoding for request().