[FR]null value

**Is your feature request related to a problem? Please describe.**
In order to delete a key in realtime db you can use either the delete method or send null as the value.
Sending `null` as a value can be beneficial sometimes because you can use the update method to delete several keys at one request.

**Describe the solution you'd like**
I would like a solution where `None` can be passed. Since requests lib does not want to change None values to null `json.dumps` can be used instead if `None` value is detected.