BigQuery: Fix Pandas DataFrame load example under Python 2.7

[plamut]

Closes #9007.

This PR adjust am example from the docs so that it works unmodified under Python 2.7, too.

How to test

• Create a BigQuery table with columns as in the screenshot posted in the issue description.
• Run the load DataFrame example from the docs using Python 2.7.

Actual result (before the fix):
The server responds with a 400 error (as reported in the issue).

Expected result (after the fix):
The adjusted example from the docs runs fine under Python 2.7, no error from the server.