Field & property overriding

Hi!
I found issue caused by field & property overriding and wrote simple workaround 
[ Code to reproduce issue ](https://github.com/Roman-Blinkov/msgpack-cli/blob/008c5c28dd16d14e232266de0fabe01df04f7c1e/test/MsgPack.UnitTest/InheritanceTest.cs) 
[ Issue fix proposal ](https://github.com/Roman-Blinkov/msgpack-cli/commit/008c5c28dd16d14e232266de0fabe01df04f7c1e)
Check #161, please.
