fix key name of gridProperties while fetching frozen column from sheet

[tekendratimsina]

clear_sheet doesn't work on current version if the sheet has frozen column as frozen column count returns default_value(0) for any sheet due to the name mismatch in gridproperties and that throws APIError.
This pr fixes the mismatched name.

Following are the key from gridProperties.

self._sheet_metadata["properties"]["gridProperties"].keys()

returns

dict_keys(['rowCount', 'columnCount', 'frozenRowCount', 'frozenColumnCount'])

[aiguofer]

awesome, thanks for your contribution!!