add RSL6a3 msgpack fixture tests

textile/features.textile

@@ -321,6 +321,7 @@ h3(#rest-channel). RestChannel
 ** @(RSL6a)@ All messages received will be decoded automatically based on the @encoding@ field and the payloads will be converted into the format they were originally sent using i.e. binary, string, or JSON
 *** @(RSL6a1)@ A set of tests must exist to ensure that the client library provides data encoding & decoding interoperability with other client libraries. The tests must use the "set of predefined interoperability message fixtures":https://github.com/ably/ably-common/blob/main/test-resources/messages-encoding.json to 1) publish a raw message to the REST API using the JSON transport and subscribe to the message using Realtime to ensure the @data@ attribute matches the fixture; 2) publish a message using the REST client library and retrieve the raw message using the history REST API using the JSON transport ensuring the @data@ matches the fixture; 3) perform the client library operation using both @JSON@ and @MsgPack@ transports. For reference, see the "Ruby":https://github.com/ably/ably-ruby/pull/94 and "iOS":https://github.com/ably/ably-cocoa/pull/459 implementations
 *** @(RSL6a2)@ A set of tests must exist to ensure that the client library provides interoperability for the @extras@ field which is a JSON-encodable object (ie a value that represents a JSON @object@ value and supports serialization to and from JSON text). The test, at a minimum, should publish a message with an @extras@ object such as @{"push":[{"title":"Testing"}]}@ and ensure it is received with an equivalent JSON-encodable object
+*** @(RSL6a3)@ A set of test should exist that ensure that the client library can successfully encode and decode binary encoded protocol-messages. The test should use the "set of predefined interoperability msgpack protocol-message fixtures":https://github.com/ably/ably-common/blob/main/test-resources/msgpack_test_fixtures.json. The msgpack field in this fixture represents the base64 encoding of the entire protocol-message, but the "data" field represents the first element of the messages field of the protocol-message.
 ** @(RSL6b)@ If, for example, incompatible encryption details are provided or invalid Base64 is detected in the message payload, an error message will be sent to the logger, but the message will still be delivered with last successful decoding and the @encoding@ field. For example, if a message had a decoding of "utf-8/cipher+aes-128-cbc/base64", and the payload was successfully Base64 decoded but the payload could not be decrypted because the @CipherParam@ details were not configured, the message would be delivered with a binary payload and an @encoding@ with the value "utf-8/cipher+aes-128-cbc". Additional steps need to be taken if decoding failed on "vcdiff" encoding; see "RTL18":#RTL18
 * @(RSL7)@ @RestChannel#setOptions@ takes a @ChannelOptions@ object and sets or updates the stored channel options, then indicates success
 * @(RSL8)@ @RestChannel#status@ function: makes a http get request to @<restHost>/channels/<channelId>@ where @<restHost>@ represents the current rest host as described by "@RSC11@":#RSC11