Column Valuers broken since #690

[randomjunk]

Issue description

Implement a struct with Valuer and use it as a named value in SQL statement.
We expect it to convert to driver.Value and work.
Our current code does this all over the place (using gorm).

The problem is that the recent change to named value handling (#690) returns a hard error if it can't convert the type, whereas it should return driver.ErrSkip to allow the default converter to handle it.

Example code

type Thing struct {
 content string
}

func (t Thing) Value() (driver.Value, error) {
  return t
}

Error log

sql: converting argument $5 type: unsupported type user.VerifyNotes, a struct

[randomjunk]

type testValuer struct {
	value string
}

func (tv testValuer) Value() (driver.Value, error) {
	return tv.value, nil
}

func TestValuer(t *testing.T) {
	runTests(t, dsn, func(dbt *DBTest) {
		in := testValuer{"a_value"}
		var out string
		var rows *sql.Rows

		dbt.mustExec("CREATE TABLE test (value VARCHAR(255)) CHARACTER SET utf8")
		dbt.mustExec("INSERT INTO test VALUES (?)", in)
		rows = dbt.mustQuery("SELECT value FROM test")
		if rows.Next() {
			rows.Scan(&out)
			if in.value != out {
				dbt.Errorf("Valuer: %v != %s", in, out)
			}
		} else {
			dbt.Errorf("Valuer: no data")
		}

		dbt.mustExec("DROP TABLE IF EXISTS test")
	})
}