normalize_entity errors when copying time index variable to new, non-time-indexed entity

[rwedge]

Bug/Feature Request Description

Using copy_variables to copy the time index column of the base entity to the new entity errors now due to how we addressed #497.

Reproducible example:

from featuretools.tests.testing_utils import make_ecommerce_entityset

es = make_ecommerce_entityset()
es.normalize_entity(base_entity_id="customers",
                    new_entity_id="cancel_reason",
                    index="cancel_reason",
                    make_time_index=False,
                    copy_variables=["signup_date"])

ValueError: DatetimeTimeIndex variable signup_date must be set using time_index parameter

We should probably change the variable type from DatetimeTimeIndex to Datetime when it is copied over (same for NumericTimeIndex / Numeric)