fix: is_field_primary_key #206

I also changed the function "is_field_primary_key"
def is_field_primary_key(self, field):
"""
Check whether key field description value can be interpreted as True
:param field:
:return:
"""
# return bool(field)
return field == 'PRI'

as it marks fileds as primary_key but some of them is not a primary key.