fix panic when merging properties (#67)

posthog.go

@@ -230,6 +230,9 @@ func (c *client) Enqueue(msg Message) (err error) {
 			}
 			m.Properties["$active_feature_flags"] = featureKeys
 		}
+		if m.Properties == nil {
+			m.Properties = NewProperties()
+		}
 		m.Properties.Merge(c.DefaultEventProperties)
 		c.setLastCapturedEvent(m)
 		msg = m

posthog_test.go

@@ -229,7 +229,33 @@ func ExampleCapture() {
 	//     }
 	//   ]
 	// }
+}
+
+func TestCaptureNoProperties(t *testing.T) {
+	defer func() {
+		// Ensure that the test doesn't panic.
+		if recover() != nil {
+			t.Error("shouldnt have panicked when merging properties into nil properties")
+		}
+	}()
+
+	_, server := mockServer()
+	defer server.Close()
 
+	client, _ := NewWithConfig("Csyjlnlun3OzyNJAafdlv", Config{
+		Endpoint:               server.URL,
+		BatchSize:              1,
+		now:                    mockTime,
+		uid:                    mockId,
+		DefaultEventProperties: NewProperties().Set("service", "api"),
+	})
+	defer client.Close()
+
+	client.Enqueue(Capture{
+		Event:            "Download",
+		DistinctId:       "123456",
+		SendFeatureFlags: false,
+	})
 }
 
 func TestEnqueue(t *testing.T) {