[MessageAutocompleteController] Register prefix when we pass attributes (#79)

Adding prefix to registeredPrefixes each time we register one with attributes

Author: JulienKode

MessageViewController/MessageAutocompleteController.swift

@@ -176,6 +176,7 @@ public final class MessageAutocompleteController: MessageTextViewListener {
     }
 
     public func registerAutocomplete(prefix: String, attributes: [NSAttributedStringKey: Any]) {
+        registeredPrefixes.insert(prefix)
         autocompleteTextAttributes[prefix] = attributes
         autocompleteTextAttributes[prefix]?[.paragraphStyle] = paragraphStyle
     }