find_column_type fix for snowflake dates

[rhaffar]

• Updated find_column_type method to only map columns types to date if they contain data with valid Snowflake date formats
• Added warehouse parameter to find_column_type method to differentiate column type mappings based on warehouse type (only affects dates)
• Updated test and internal calls of find_column_type

Fixes #221

[CLAassistant]

All committers have signed the CLA.

[fdosani]

LGTM. Was thinking through the logic since it has been a while since I went through this stuff. Overall it looks good. thanks for updating the unit tests and adding in extras there.

[rhaffar]

LGTM. Was thinking through the logic since it has been a while since I went through this stuff. Overall it looks good. thanks for updating the unit tests and adding in extras there.

Thanks for reviewing Faisal! Any idea how long the Whitesource checks tend to take? Can't merge until they are done.