fix #64 set databaseId before parse mapper xmls

when parse mapper xml files , it needs to know what databaseId is。
org.apache.ibatis.builder.xml.XMLConfigBuilder'smethod:databaseIdProviderElement,will set databaseId if "environment != null && databaseIdProvider != null",but the configuration's environment  is null becase it will set after  "xmlConfigBuilder.parse();" in file "SqlSessionFactoryBean.java"

Author: ylyxf

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

@@ -425,7 +425,15 @@ protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
         }
       }
     }
-
+    
+    if (this.databaseIdProvider != null) {//fix #64 set databaseId before parse mapper xmls
+      try {
+        configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
+      } catch (SQLException e) {
+        throw new NestedIOException("Failed getting a databaseId", e);
+      }
+    }
+    
     if (xmlConfigBuilder != null) {
       try {
         xmlConfigBuilder.parse();
@@ -446,14 +454,6 @@ protected SqlSessionFactory buildSqlSessionFactory() throws IOException {
 
     configuration.setEnvironment(new Environment(this.environment, this.transactionFactory, this.dataSource));
 
-    if (this.databaseIdProvider != null) {
-      try {
-        configuration.setDatabaseId(this.databaseIdProvider.getDatabaseId(this.dataSource));
-      } catch (SQLException e) {
-        throw new NestedIOException("Failed getting a databaseId", e);
-      }
-    }
-
     if (!isEmpty(this.mapperLocations)) {
       for (Resource mapperLocation : this.mapperLocations) {
         if (mapperLocation == null) {