Add specs on Module#const_get raising a NameError with missing constant.

NameError should be raised with the missing constant as the second
argument, so it would show up when calling `#name` on the error.
Module#const_get: raise NameError with missing constant name.
Merge pull request #2424 from razielgn/feature-module-const_get-name_error-const-name

Module#const_get should raise NameError with missing constant attached.