'Returns an string value from a v1 Value.
Args:
v1_value: an googledatastore.Value
Returns:
a string
Throws:
InvalidConversionError: if the value doesn\'t contain a string value'

def __get_v1_string_value(self, v1_value):

    check_conversion(v1_value.HasField('string_value'), 'Value    does    not    contain    a    string    value.')
    return v1_value.string_value
