fix RichValue#as

to enable parametrized type to be converted from value using RichValue#as

e.g. `readAsValue(writeV((1,2))).as[(Int,Int)]` doesn't work now. 

NOTE: This commit requires the commit of msgpack-java (choplin/msgpack-java@50d77092c045298267ffb822b338a469aa3ea70e).
A pull request for this commit has already been opend, but not merged yet.