replace `clazz` to `klass` so to uniformize the variable name

This pull request might be about coding style.
I looked into entire source and found that there are two variable names which are `clazz` and `klass` are used  to express the class.
Since ruby prefers `klass` as the variable name, I think it makes code more simple if we replace the `clazz` to `klass` and uniformizing it.
Is this a good idea?