Fix Open() journal mode regression

[why]
see https://github.com/mattn/go-sqlite3/issues/607

SQLite default journal mode is DELETE, but forcing it on open causes "database is locked"
if other connection exists with WAL mode, for example.

[how]
Don't set DELETE mode if not set in DSN explicitly.

[testing]
Run tests in my project where WAL mode is used.

sqlite3.go

@@ -1000,7 +1000,7 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 	deferForeignKeys := -1
 	foreignKeys := -1
 	ignoreCheckConstraints := -1
-	journalMode := "DELETE"
+	var journalMode string
 	lockingMode := "NORMAL"
 	queryOnly := -1
 	recursiveTriggers := -1
@@ -1571,11 +1571,12 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 	}
 
 	// Journal Mode
-	// Because default Journal Mode is DELETE this PRAGMA can always be executed.
+	if journalMode != "" {
 		if err := exec(fmt.Sprintf("PRAGMA journal_mode = %s;", journalMode)); err != nil {
 			C.sqlite3_close_v2(db)
 			return nil, err
 		}
+	}
 
 	// Locking Mode
 	// Because the default is NORMAL and this is not changed in this package