Can't certify data source while publishing

[elsherif]

Describe the bug
I created a new DatasourceItem and set the certification attributes correctly. When publishing the data source, it can be published successfully, but wouldn't be certified. In logs I can see that TSC is sending the tsRequest with out the certification information.

Versions
Details of your environment, including:

• Tableau Online
• Python version: 3.8
• TSC library version: 0.17.0

To Reproduce
publish_mode = tab_server.PublishMode.Overwrite
project_id = ...
connection_credentials = ...
filepath = ...
new_datasource = TSC.DatasourceItem(project_id)

new_datasource.certified = True
new_datasource.certification_note = "note"

new_datasource = tab_server.datasources.publish(
new_datasource,
filepath,
publish_mode,
connection_credentials=connection_credentials,
)

Results
Data source published but not certified.

[jacalata]

see if I can fix this with the general clean-up of connections v credentials in #1057