BigQuery pre-aggregations datetime error

Hi!
Cube (1.2.9) generates SQL to pre-aggregated table but doesn’t cast field to datetime
when we use filter by this member.

```
pre_aggregations: PRE_AGGREGATIONS_ENABLED ? {
model__daily_analysis_rollup_by_activity: {
dimensions: (CUBE) => [
CUBE.app_name,
CUBE.cohort_date,

],
measures: (CUBE) => [
CUBE.count,
],

time_dimensions: [
{ dimension: (CUBE) => CUBE.cohort_date, granularity: 'day' },
{ dimension: (CUBE) => CUBE.cohort_date, granularity: 'week' },
{ dimension: (CUBE) => CUBE.cohort_date, granularity: 'month' },
{ dimension: (CUBE) => CUBE.cohort_date, granularity: 'quarter' },
{ dimension: (CUBE) => CUBE.cohort_date, granularity: 'year' },
],
```