upgrade undertow to 2.0.21.Final

Author: stevehu

pom.xml

@@ -68,7 +68,7 @@
         <version.junit>4.12</version.junit>
         <version.mockito>2.7.21</version.mockito>
         <version.hamcrest>1.3</version.hamcrest>
-        <version.undertow>2.0.16.Final</version.undertow>
+        <version.undertow>2.0.21.Final</version.undertow>
     </properties>
     <dependencies>
         <dependency>