union should explicitly specify table name in casts

Bigquery gets confused by tables that have a column with the same name in CASTs, and will try to cast the entire table instead of the column.

```
-- causes error, as foo is a struct
select cast(`foo` as STRING) as `foo`
from `data-playground-1234`.`myschema`.`foo`

-- works when table name is specified
select cast(`foo`.`foo` as STRING) as `foo`
from `data-playground-1234`.`myschema`.`foo`
```

I *think* this is a simple as adding `table.name` [here](https://github.com/fishtown-analytics/dbt-utils/blob/master/macros/sql/union.sql#L61), like:

``` {%- set col_name = adapter.quote(table.name) + '.' + adapter.quote(col_name) if col_name in table_columns[table] else 'null' %}```