Fix prefix query with case insensitive setting

The `PrefixQuery` query wasn't serialized correctly if only name,
prefix, and case-sensitivity were set.

Close #1546

Author: olivere

search_queries_prefix.go

@@ -52,7 +52,7 @@ func (q *PrefixQuery) Source() (interface{}, error) {
 	query := make(map[string]interface{})
 	source["prefix"] = query
 
-	if q.boost == nil && q.rewrite == "" && q.queryName == "" {
+	if q.boost == nil && q.rewrite == "" && q.queryName == "" && q.caseInsensitive == nil {
 		query[q.name] = q.prefix
 	} else {
 		subQuery := make(map[string]interface{})

search_queries_prefix_test.go

@@ -26,6 +26,23 @@ func TestPrefixQuery(t *testing.T) {
 	}
 }
 
+func TestPrefixQueryWithCaseInsensitive(t *testing.T) {
+	q := NewPrefixQuery("user", "ki").CaseInsensitive(true)
+	src, err := q.Source()
+	if err != nil {
+		t.Fatal(err)
+	}
+	data, err := json.Marshal(src)
+	if err != nil {
+		t.Fatalf("marshaling to JSON failed: %v", err)
+	}
+	got := string(data)
+	expected := `{"prefix":{"user":{"case_insensitive":true,"value":"ki"}}}`
+	if got != expected {
+		t.Errorf("expected\n%s\n,got:\n%s", expected, got)
+	}
+}
+
 func TestPrefixQueryWithOptions(t *testing.T) {
 	q := NewPrefixQuery("user", "ki").CaseInsensitive(true)
 	q = q.QueryName("my_query_name")