Add servletRelativeAction form tag attribute

A recent change in FormTag to prepend the context and servlet paths if
not present, causes issues when used in portlet applications.
This change introduces a servletRelativeAction form tag attribute that
must be used for the context and servlet paths to be prepended.

Issue: SPR-10382

spring-webmvc/src/test/java/org/springframework/web/servlet/tags/form/FormTagTests.java

@@ -179,7 +179,7 @@ public class FormTagTests extends AbstractHtmlElementTagTests {
 		String onreset = "onreset";
 
 		this.tag.setCommandName(commandName);
-		this.tag.setAction(action);
+		this.tag.setServletRelativeAction(action);
 		this.tag.setMethod(method);
 		this.tag.setEnctype(enctype);
 		this.tag.setOnsubmit(onsubmit);