[boost] [mpl] Fix to Ticket #11001

I just finished a pull request https://github.com/boostorg/mpl/pull/14 with a fix for Ticket #11001 https://svn.boost.org/trac/boost/ticket/11001, which, by the way, I reported myself, so I'm quite confident it's fixed. Could one of the maintainers kindly review it? Oh and it is worth it to mention that I also included a new test case to ensure the fix is working. Kind regards, *Bruno C. O. Dutra*