Allow decoding raw strings

[jasonpaulos]

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This PR adds the ability to skip UTF-8 decoding of strings when you enable the rawStrings decoding option.

This is useful because I have encountered some encoded msgpack strings which do not contain valid UTF-8 data, and without a feature like this, it would be impossible to decode them properly.

According to the msgpack spec, this ability is encouraged:

• String objects may contain invalid byte sequence and the behavior of a deserializer depends on the actual implementation when it received invalid byte sequence
  • Deserializers should provide functionality to get the original byte array so that applications can decide how to handle the object