fix(singleton class): Fix singleton class as a injectable class

When providing singleton class as a dependency, we tried to calculate the deepest dependency scope of it and failed due to the class callable is not the function but the class itself.
refactor(test): Merge test together to improve readability
doc(test): Change test name to be more descriptive
Merge pull request #28 from yozo1984/master

fix(singleton class): Fix singleton class as a injectable class