Literal expressions fail to parse when returned from Postgres query

Hey again!

Running this:

```haskell
import Database.Beam
import Database.Beam.Postgres
import Database.Beam.Postgres.Syntax
import Data.Text (Text)

test :: Connection -> IO ()
test conn = do
let
t :: QExpr PgExpressionSyntax s Text
t = "abc"

result <- withDatabaseDebug putStrLn conn (runSelectReturningOne (select (pure t)) :: Pg (Maybe Text))
print result
```

results in:

```sql
SELECT 'abc' AS "res0"
testapp: PgRowParseError (PgRowCouldNotParseField 0)
```

It seems like the error can be fixed by casting the value to the right type, i.e. by making the query `SELECT ('abc'::text) AS "res0"`. I suppose the parser (which might be inherited from postgres-simple?) is strict about what types it allows whereas the printer outputs string literals which have unknown type until you cast them (`pg_typeof('abc')` is `unknown`).

Several (most?) other types also have this problem.

Cheers!