BigQuery: SchemaField cannot load resource with missing `mode` property

[tswast]

The mode property is optional in the REST API, but this code fails:

SchemaField.from_api_repr({'name': 'colname', 'type': 'INT64'})

I believe this is due to the line

mode=api_repr['mode'].upper(),

which does not properly handle when the mode property is missing.

Related to #3723 as it is often convenient to omit the mode parameter when defining a schema file by hand.