Disabling allow_explicit_index breaks bulk

fixes elastic#4668

Author: kimchy

src/main/java/org/elasticsearch/action/bulk/BulkRequest.java

@@ -341,9 +341,6 @@ public BulkRequest add(BytesReference data, boolean contentUnsafe, @Nullable Str
                     // we use internalAdd so we don't fork here, this allows us not to copy over the big byte array to small chunks
                     // of index request. All index requests are still unsafe if applicable.
                     if ("index".equals(action)) {
-                        if (!allowExplicitIndex) {
-                            throw new ElasticSearchIllegalArgumentException("explicit index in bulk is not allowed");
-                        }
                         if (opType == null) {
                             internalAdd(new IndexRequest(index, type, id).routing(routing).parent(parent).timestamp(timestamp).ttl(ttl).version(version).versionType(versionType)
                                     .source(data.slice(from, nextMarker - from), contentUnsafe)

src/test/java/org/elasticsearch/action/bulk/BulkRequestTests.java

@@ -28,7 +28,6 @@
 import org.junit.Test;
 
 import static org.elasticsearch.common.io.Streams.copyToStringFromClasspath;
-import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 
@@ -80,4 +79,17 @@ public void testSimpleBulk4() throws Exception {
         assertThat(((UpdateRequest) bulkRequest.requests().get(1)).upsertRequest().source().toUtf8(), equalTo("{\"counter\":1}"));
     }
 
+    @Test
+    public void testBulkAllowExplicitIndex() throws Exception {
+        String bulkAction = copyToStringFromClasspath("/org/elasticsearch/action/bulk/simple-bulk.json");
+        try {
+            new BulkRequest().add(new BytesArray(bulkAction.getBytes(Charsets.UTF_8)), true, null, null, false);
+            assert false;
+        } catch (Exception e) {
+
+        }
+
+        bulkAction = copyToStringFromClasspath("/org/elasticsearch/action/bulk/simple-bulk5.json");
+        new BulkRequest().add(new BytesArray(bulkAction.getBytes(Charsets.UTF_8)), true, "test", null, false);
+    }
 }

src/test/java/org/elasticsearch/action/bulk/simple-bulk5.json

@@ -0,0 +1,8 @@
+{ "index": {
+    "_type": "type1",
+    "_id": "1"
+} }
+{ "field1" : "value1" }
+{ "delete" : { "_type" : "type1", "_id" : "2" } }
+{ "create" : { "_type" : "type1", "_id" : "3" } }
+{ "field1" : "value3" }