Update ScriptTemplateView to manage content type

This commit introduces the following changes:
 - Content type can now be properly configured
 - Default content type is "text/html"
 - Content type and charset are now properly set in the response

Issue: SPR-13379

spring-webmvc/src/test/resources/org/springframework/web/servlet/config/mvc-config-view-resolution.xml

@@ -34,7 +34,8 @@
 
 	<mvc:groovy-configurer resource-loader-path="/test" cache-templates="false" auto-indent="true" />
 
-	<mvc:script-template-configurer engine-name="nashorn" render-function="render" charset="ISO-8859-1"
+	<mvc:script-template-configurer engine-name="nashorn" render-function="render"
+									content-type="text/plain" charset="ISO-8859-1"
 									resource-loader-path="classpath:" shared-engine="false">
 		<mvc:script location="org/springframework/web/servlet/view/script/nashorn/render.js" />
 	</mvc:script-template-configurer>