Add explicit status response to OPTIONS handler

Write the http.StatusOK header in the OPTIONS
handler and update the unit tests to refer to the
response code using the const from the http package.

api.go

@@ -793,6 +793,7 @@ func createRouter(srv *Server, logging bool) (*mux.Router, error) {
 		if srv.enableCors {
 			writeCorsHeaders(w, r)
 		}
+		w.WriteHeader(http.StatusOK)
 	})
 	return r, nil
 }

api_test.go

@@ -1260,7 +1260,7 @@ func TestOptionsRoute(t *testing.T) {
 	}
 
 	router.ServeHTTP(r, req)
-	if r.Code != 200 {
+	if r.Code != http.StatusOK {
 		t.Errorf("Expected response for OPTIONS request to be \"200\", %v found.", r.Code)
 	}
 }
@@ -1287,7 +1287,7 @@ func TestGetEnabledCors(t *testing.T) {
 	}
 
 	router.ServeHTTP(r, req)
-	if r.Code != 200 {
+	if r.Code != http.StatusOK {
 		t.Errorf("Expected response for OPTIONS request to be \"200\", %v found.", r.Code)
 	}