Enable assertions in integration tests

When starting a standalone cluster, we do not able assertions. This is
problematic because it means that we miss opportunities to catch
bugs. This commit enables assertions for standalone integration tests,
and fixes a couple bugs that were uncovered by enabling these.

Relates #22334

Author: jasontedor

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy

@@ -72,9 +72,11 @@ class ClusterConfiguration {
     boolean useMinimumMasterNodes = true
 
     @Input
-    String jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
-            " " + "-Xmx" + System.getProperty('tests.heap.size', '512m') +
-            " " + System.getProperty('tests.jvm.argline', '')
+    String jvmArgs = "-ea" +
+        " " + "-Xms" + System.getProperty('tests.heap.size', '512m') +
+        " " + "-Xmx" + System.getProperty('tests.heap.size', '512m') +
+        " " + System.getProperty('tests.jvm.argline', '')
+
 
     /**
      * A closure to call which returns the unicast host to connect to for cluster formation.

core/src/main/java/org/elasticsearch/rest/BytesRestResponse.java

@@ -89,9 +89,10 @@ public BytesRestResponse(RestChannel channel, RestStatus status, Exception e) th
             this.content = BytesArray.EMPTY;
             this.contentType = TEXT_CONTENT_TYPE;
         } else {
-            XContentBuilder builder = convert(channel, status, e);
-            this.content = builder.bytes();
-            this.contentType = builder.contentType().mediaType();
+            try (final XContentBuilder builder = convert(channel, status, e)) {
+                this.content = builder.bytes();
+                this.contentType = builder.contentType().mediaType();
+            }
         }
         if (e instanceof ElasticsearchException) {
             copyHeaders(((ElasticsearchException) e));

core/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetMappingAction.java

@@ -75,8 +75,10 @@ public RestResponse buildResponse(GetMappingsResponse response, XContentBuilder
                     if (indices.length != 0 && types.length != 0) {
                         return new BytesRestResponse(OK, builder.startObject().endObject());
                     } else if (indices.length != 0) {
+                        builder.close();
                         return new BytesRestResponse(channel, new IndexNotFoundException(indices[0]));
                     } else if (types.length != 0) {
+                        builder.close();
                         return new BytesRestResponse(channel, new TypeMissingException("_all", types[0]));
                     } else {
                         return new BytesRestResponse(OK, builder.startObject().endObject());