createInstantMessage abstract method
Creates a new InstantMessage from envelope and plaintext content.
Combines routing metadata (envelope) with unencrypted content to form a complete instant message (plaintext stage).
head is the message envelope (routing metadata, cannot be null).
body is the plaintext content (message payload, cannot be null).
Returns a new InstantMessage instance.
Implementation
InstantMessage createInstantMessage(Envelope head, Content body);