fixed issue #781 , upgrade fastsql 540

pom.xml

@@ -247,7 +247,7 @@
             <dependency>
                 <groupId>com.alibaba.fastsql</groupId>
                 <artifactId>fastsql</artifactId>
-                <version>2.0.0_preview_538</version>
+                <version>2.0.0_preview_540</version>
             </dependency>
             <dependency>
                 <groupId>com.alibaba</groupId>