Encodes an instance of Encodable and overwrites the encoded data
to the path referred by this DatabaseReference. If no value exists,
it is created. If a value already exists, it is overwritten.
See Database.Encoder for more details about the encoding process.
An instance of Encodable to be encoded to a document.
encoder
An encoder instance to use to run the encoding.
completion
A block to execute once the value has been successfully
written to the server. This block will not be called while
the client is offline, though local changes will be visible
immediately.