add the synchronized keyword to getDriver()

Author: donhui

src/main/java/org/apache/ibatis/datasource/unpooled/UnpooledDataSource.java

@@ -136,7 +136,7 @@ public void setDriverProperties(Properties driverProperties) {
     this.driverProperties = driverProperties;
   }
 
-  public String getDriver() {
+  public synchronized String getDriver() {
     return driver;
   }