bug fixed for mysql connection validation checker, and improved odps sql formatter

src/main/java/com/alibaba/druid/sql/ast/expr/SQLCharExpr.java

@@ -15,6 +15,7 @@
  */
 package com.alibaba.druid.sql.ast.expr;
 
+import com.alibaba.druid.sql.SQLUtils;
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;
 
 public class SQLCharExpr extends SQLTextLiteralExpr implements SQLValuableExpr{
@@ -47,4 +48,8 @@ public class SQLCharExpr extends SQLTextLiteralExpr implements SQLValuableExpr{
     public Object getValue() {
         return this.text;
     }
+    
+    public String toString() {
+        return SQLUtils.toSQLString(this);
+    }
 }

src/main/java/com/alibaba/druid/sql/ast/expr/SQLIdentifierExpr.java

@@ -97,4 +97,7 @@ public class SQLIdentifierExpr extends SQLExprImpl implements SQLName {
         return true;
     }
 
+    public String toString() {
+        return this.name;
+    }
 }