Unmarshalled Strings Unsafely Share Byte Slices

[swdunlop]

If the bytes in the byte slice passed to msgpack.Unmarshal change after msgpack.Unmarshal returns, any of the unmarshaled strings may change their contents, violating the expectation that Go strings are immutable. This is probably due to the use of unsafe.Pointer in decoder.asString. This is surprising behavior which can really screw things up in Go if the original byte slice was a buffer whose contents are replaced on each message.

See https://play.golang.org/p/YQyb8ie8Qqd for a simple repro of the issue.

The simplest fix I can see is to document this behavior and warn not to reuse the underlying byte array until any strings are expunged. Discontinuing this behavior would likely have a dramatic negative impact on performance. The most complex fix would be to make the behavior of asString configurable, which would let the users decide if they want to sacrifice sanity for efficiency. (And this is an insane behavior -- consider what happens if the resulting string was used as a map key by an unsuspecting user.)

[shamaton]

@swdunlop
Sorry for delay.
Thank you for proposing this issue and sharing sample code!!
I understood how this behavior.

Performance is important, but being safe to use is more important.
So, I updated behavior by this PR. #27

Performance is less, but all users can use this package safe completely.