Add test for using fuzziness parameter in multi_match query

There was an issue with using fuzziness parameter in multi_match query that has
been reported in #18710 and was fixed in Lucene 6.2 that is now used on master.
In order to verify that fix and close the original issue this PR adds the test
from that issue as an integration test.

core/src/test/java/org/elasticsearch/search/query/MultiMatchQueryIT.java

@@ -635,6 +635,43 @@ public class MultiMatchQueryIT extends ESIntegTestCase {
         assertFirstHit(searchResponse, hasId("ultimate1"));
     }
 
+    /**
+     * Test for edge case where field level boosting is applied to field that doesn't exist on documents on
+     * one shard. There was an issue reported in https://github.com/elastic/elasticsearch/issues/18710 where a
+     * `multi_match` query using the fuzziness parameter with a boost on one of two fields returns the
+     * same document score if both documents are placed on different shard. This test recreates that scenario
+     * and checks that the returned scores are different.
+     */
+    public void testFuzzyFieldLevelBoosting() throws InterruptedException, ExecutionException {
+        String idx = "test18710";
+        CreateIndexRequestBuilder builder = prepareCreate(idx).setSettings(Settings.builder()
+                .put(indexSettings())
+                .put(SETTING_NUMBER_OF_SHARDS, 3)
+                .put(SETTING_NUMBER_OF_REPLICAS, 0)
+                );
+        assertAcked(builder.addMapping("type", "title", "type=string", "body", "type=string"));
+        ensureGreen();
+        List<IndexRequestBuilder> builders = new ArrayList<>();
+        builders.add(client().prepareIndex(idx, "type", "1").setSource(
+                "title", "foo",
+                "body", "bar"));
+        builders.add(client().prepareIndex(idx, "type", "2").setSource(
+                "title", "bar",
+                "body", "foo"));
+        indexRandom(true, false, builders);
+
+        SearchResponse searchResponse = client().prepareSearch(idx)
+                .setExplain(true)
+                .setQuery(multiMatchQuery("foo").field("title", 100).field("body")
+                        .fuzziness(0)
+                        ).get();
+        SearchHit[] hits = searchResponse.getHits().getHits();
+        assertNotEquals("both documents should be on different shards", hits[0].getShard().getShardId(), hits[1].getShard().getShardId());
+        assertEquals("1", hits[0].getId());
+        assertEquals("2", hits[1].getId());
+        assertThat(hits[0].getScore(), greaterThan(hits[1].score()));
+    }
+
     private static void assertEquivalent(String query, SearchResponse left, SearchResponse right) {
         assertNoFailures(left);
         assertNoFailures(right);