gbq.io.to_gbq raises InvalidSchema even thought schema matches
DF's schema returned by pandas.io.gbq.generate_bq_schema(df):
```
{'fields': [
{'type': 'TIMESTAMP', 'name': 'field1'}, 
{'type': 'STRING', 'name': 'field2'}, 
{'type': 'FLOAT', 'name': 'field3'}, 
{'type': 'STRING', 'name': 'field4'},
{'type': 'FLOAT', 'name': 'field5'},
{'type': 'STRING', 'name': 'field6'}, 
{'type': 'FLOAT', 'name': 'field7'}, 
{'type': 'STRING', 'name': 'field8'},
{'type': 'STRING', 'name': 'field9'}]}
```

Schema from BQ CLI:
```
|- field1: timestamp
|- field2: string
|- field3: float
|- field4: string
|- field5: float
|- field6: string
|- field7: float
|- field8: string
|- field9: string
```

`to_gbq(df,"dataset.table","project",if_exists="append",private_key=service_key_path)`
Throws:

```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/lib/python2.7/site-packages/pandas/io/gbq.py", line 827, in to_gbq
    raise InvalidSchema("Please verify that the structure and "
pandas.io.gbq.InvalidSchema: Please verify that the structure and data types in the DataFrame match the schema of the destination table.
```