`DECIMAL` value in `union` query loses precision and scale information

In MySQL:
```
mysql> with recursive t (n) as (select sum(1) from dual union all select (2.00) from dual) select sum(n) from t;
+--------+
| sum(n) |
+--------+
| 3.00 |
+--------+
1 row in set (0.00 sec)
```
In Dolt:
```
mydb1> with recursive t (n) as (select sum(1) from dual union all select (2.00) from dual) select sum(n) from t;
+--------+
| sum(n) |
+--------+
| 3 |
+--------+
```

This is from `mergeUnionSchemas` rule, decimal type is cast to string.