Do not play with mutable data

[nogenhat]

There can be issues, when you force encode original string into binary and then someone else want to use this string.

[fl00r]

I believe the problem is here https://github.com/protobuf-ruby/beefcake/blob/71b06dc47b7284f924323bbbb0941d438eb0eb6e/lib/beefcake/buffer/encode.rb#L107

[bkerley]

Is there a way to write a test for this?

[bkerley]

This is closely related to but distinct from #69 and #70 .

That is all :)