Inconsistent encoding/decoding of Utf8 text

Thank you for maintaining this fantastic library, 

I have found an inconsistency when encoding and decoding 
"\55296". 

```haskell
> import Data.Text.Encoding
> :set -XOverloadedStrings
> decodeUtf8 (encodeUtf8 "\55296")
"\65533"
```

The bug seems to be system independent and happening in at least version 1.2.2.2.

