[spring] fix for http://code.google.com/p/mybatis/issues/detail?id=548 . Respect databaseId if there was already one in mybatis-config.xml

Author: emacarron

src/main/java/org/mybatis/spring/SqlSessionFactoryBean.java

@@ -390,7 +390,7 @@ protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
     Environment environment = new Environment(this.environment, this.transactionFactory, this.dataSource);
     configuration.setEnvironment(environment);
 
-    if (this.databaseIdProvider != null) {
+    if (configuration.getDatabaseId() == null && this.databaseIdProvider != null) {
       try {
         configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
       } catch (SQLException e) {