Fix duplicate header read in ConvertToJson in ext format

[neuecc]

writer.WriteExtensionFormat(new ExtensionResult(47, new byte[] { 1, 10, 100 }));
writer.Flush();
// exception
var str = MessagePackSerializer.ConvertToJson(sequence.AsReadOnlySequence);

throws exception when call ConvertToJson when target block is ext format.

because it read extension header twice.

var header = reader.ReadExtensionFormatHeader();
var ext = reader.ReadExtensionFormat();

This PR fixed it and add tests.

[AArnott]

This looks good. But since it fixes a 2.0 bug, shouldn't it target the v2.0 branch?

[neuecc]

surely.surely.
I've derived from 2.0 and force pushed.