Don't set message encoding for valid utf-8 string data

[lmars]

See explanation in ably/specification#28.

[sacOO7]

@lmars , seems utf8 encoding is also used at one more place. Also, prev. comment has been added as to no idea why it's there.
Can you check if it's needed

ably-go/ably/proto_message.go

Lines 87 to 91 in 4c447dc

	if _, ok := m.Data.(string); ok {
	// Not sure why this is useful, and can't find the requirement in the
	// spec, but test fixtures expect this.
	m.Encoding = mergeEncoding(m.Encoding, encUTF8)
	}

[sacOO7]

Also, we can remove this //todo comment, since proto is merged into ably

ably-go/ably/proto_message.go

Line 42 in 4c447dc

// TODO: Unexport once proto gets merged into package ably.

[lmars]

@sacOO7 thanks, I've clarified the requirement to add utf-8 encoding to encrypted strings in 99a3f68, and removed that outdated comment in a57270a.

[sacOO7]

@lmars , not sure but I feel when we are using cipher, we are adding extra layer of encoding for byte [] type, which increases payload size (33 %, goes exponential for next encoding iterations). I was thinking maybe you can improve on that one/ cut the part of code that does that when cipher is enabled. Although, this is not part of the fix. but I just feel we can include it as a part of fix

[lmars]

@sacOO7

this is not part of the fix. but I just feel we can include it as a part of fix

Yes, so whilst what you describe could potentially be a problem, let's deal with that separately, the priority here is fixing the behaviour we know is broken. If you're happy that we've fixed the reported issue, please approve this PR so we can merge and release it.

[sacOO7]

Fixes incomplete #107