TST: Potentially add a unit test to verify the functionality of parameter auth_local_webserver

[parthea]

It may be possible to add a unit test to verify the functionality of parameter auth_local_webserver of read_gbq() and to_gbq() in #39 in order to improve testing. One potential solution is to capture the output in the unit test and look for specific text :

'Go to the following link in your browser:' , when auth_local_webserver=False
or
'Your browser has been opened to visit:' , when auth_local_webserver=True

[tswast]

This parameter is now passed on through to pydata-google-auth. I've filed this as pydata/pydata-google-auth#9 over there.