Allow Mu to be the result of the Promise
Allow Mu to be the result of the Promise

Otherwise, the tests are failing after 627dcf4
See http://irclog.perlgeek.de/perl6-dev/2016-06-18#i_12689953
for discussion

b2gills/rakudo,samcv/rakudo,usev6/rakudo,b2gills/rakudo,tbrowder/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,samcv/rakudo,Gnouc/rakudo,jonathanstowe/rakudo,Gnouc/rakudo,nbrown/rakudo,softmoth/rakudo,jonathanstowe/rakudo,tony-o/rakudo,rakudo/rakudo,nbrown/rakudo,Gnouc/rakudo,salortiz/rakudo,awwaiid/rakudo,tony-o/rakudo,rakudo/rakudo,tbrowder/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,nbrown/rakudo,tbrowder/rakudo,salortiz/rakudo,raydiak/rakudo,awwaiid/rakudo,labster/rakudo,labster/rakudo,softmoth/rakudo,rakudo/rakudo,tbrowder/rakudo,jonathanstowe/rakudo,awwaiid/rakudo,jonathanstowe/rakudo,awwaiid/rakudo,MasterDuke17/rakudo,sjn/rakudo,usev6/rakudo,tony-o/rakudo,salortiz/rakudo,salortiz/rakudo,awwaiid/rakudo,tbrowder/rakudo,zostay/rakudo,samcv/rakudo,sjn/rakudo,b2gills/rakudo,MasterDuke17/rakudo,rakudo/rakudo,labster/rakudo,sjn/rakudo,nbrown/rakudo,softmoth/rakudo,sjn/rakudo,tony-o/rakudo,lucasbuchala/rakudo,rakudo/rakudo,lucasbuchala/rakudo,lucasbuchala/rakudo,salortiz/rakudo,softmoth/rakudo,usev6/rakudo,tony-o/rakudo,usev6/rakudo,zostay/rakudo,raydiak/rakudo,b2gills/rakudo,Gnouc/rakudo,sjn/rakudo,labster/rakudo,nbrown/rakudo,samcv/rakudo,tony-o/rakudo,tbrowder/rakudo,samcv/rakudo,zostay/rakudo,softmoth/rakudo,rakudo/rakudo,raydiak/rakudo,usev6/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,labster/rakudo,raydiak/rakudo,jonathanstowe/rakudo,b2gills/rakudo,zostay/rakudo,salortiz/rakudo,nbrown/rakudo,labster/rakudo