yeah rvm 1.24.7, ruby, ruby-1.9.3-p429.
I did not look into the tests but my expectation would be, that installation is not covered by tests. When I searched the archive it seems like testing infrastructure is leveraging Gemfile, not rake which is (maybe/probably?) used for generating the actual gem.
If you look at Gemfile you can see that multi_json is spelled there correctly which would explain why it does not fail during set up of the testing environment. Is there a specific reason why there are 2 different ways how to specify dependencies?