[SPARK-24188][CORE] Restore "/version" API endpoint.

It was missing the jax-rs annotation.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #21245 from vanzin/SPARK-24188.

Change-Id: Ib338e34b363d7c729cc92202df020dc51033b719

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/status/api/v1/ApiRootResource.scala

@@ -49,6 +49,7 @@ private[v1] class ApiRootResource extends ApiRequestContext {
   @Path("applications/{appId}")
   def application(): Class[OneApplicationResource] = classOf[OneApplicationResource]
 
+  @GET
   @Path("version")
   def version(): VersionInfo = new VersionInfo(org.apache.spark.SPARK_VERSION)
 

core/src/test/scala/org/apache/spark/deploy/history/HistoryServerSuite.scala

@@ -296,6 +296,11 @@ class HistoryServerSuite extends SparkFunSuite with BeforeAndAfter with Matchers
     all (siteRelativeLinks) should startWith (uiRoot)
   }
 
+  test("/version api endpoint") {
+    val response = getUrl("version")
+    assert(response.contains(SPARK_VERSION))
+  }
+
   test("ajax rendered relative links are prefixed with uiRoot (spark.ui.proxyBase)") {
     val uiRoot = "/testwebproxybase"
     System.setProperty("spark.ui.proxyBase", uiRoot)