Selecting custom fields works differently by the method of specifying columns

When I have a type decorator-based column in a SQLAlchemy table, querying those columns with explicit syntax works as expected (values are converted back and forth through the type decorator), but when using raw SQL syntax (e.g., `"*"`) it does not.

The following examples go through the type decorator as expected:
```
query = sa.select([mytable]).select_from(mytable)
await connection.execute(query)
```
```
query = sa.select([mytable.c.col1, mytable.c.col2]).select_from(mytable)
await connection.execute(query)
```

The following example does *not* go through the type decorator:
```
query = sa.select("*").select_from(mytable)
await connection.execute(query)
```

This has caused many hidden bugs for my case.
Is this an expected behavior, an upstream issue in SQLAlchemy, or a potential bug in aiopg?