Special case the underlying type of []byte

Underlying types were already tried. But []byte is not a normal
underlying type. It is a slice. But since is can be treated as a scalar
instead of an array / slice we need to special case it.

https://github.com/jackc/pgx/issues/1763

pgtype/pgtype_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"database/sql/driver"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"net"
@@ -417,6 +418,14 @@ func TestMapEncodeByteSliceIntoUnregisteredTypeTextFormat(t *testing.T) {
 	require.Equal(t, []byte(`\x00010203`), buf)
 }
 
+// https://github.com/jackc/pgx/issues/1763
+func TestMapEncodeNamedTypeOfByteSliceIntoTextTextFormat(t *testing.T) {
+	m := pgtype.NewMap()
+	buf, err := m.Encode(pgtype.TextOID, pgtype.TextFormatCode, json.RawMessage(`{"foo": "bar"}`), nil)
+	require.NoError(t, err)
+	require.Equal(t, []byte(`{"foo": "bar"}`), buf)
+}
+
 // https://github.com/jackc/pgx/issues/1326
 func TestMapScanPointerToRenamedType(t *testing.T) {
 	srcBuf := []byte("foo")