feat: support DateTime and Uri

[amondnet]

Using DateTime and Uri will throw ArgumentError.

ArgumentError: Invalid argument: Instance of 'DateTime'

This pull request allows us to use dart DateTime and Uri.

_mixpanel.track("Track event with property", properties: {
                  "Cool Property": "Property Value",
                  "test": 233,
                  "complex": {
                    "child": [
                      {"deep1": "value1"},
                      {
                        "deep2": [1, 2]
                      }
                    ]
                  },
                  "date": DateTime.now(),
                  "uri": Uri.parse("https://mixpanel.com")
                })

[zihejia]

Thanks @amondnet !