fix gcc warning when CXXFLAGS contains '-Wextra'

When CXXFLAGS contains '-Wextra', as I set 
SET(CMAKE_CXX_FLAGS_RELEASE ‘-Wall -Wextra’)
in root CMakeLists.txt.
I got the following error:
json_value.cpp:179:26: warning: enumeral and non-enumeral type in conditional expression [enabled by default]

I have describe it in the closed pull as follows:
https://github.com/open-source-parsers/jsoncpp/pull/80

Thanks the help of Christopher Dunn 
"
Yours will be slightly less efficient. I know this CZString stuff is kind of hacky, but why not use static_cast<ArrayIndex>(other.index_ == noDuplication ? noDuplication : duplicate) after the question mark?
"
I test the modification and it works fine.