Write specs for StopIteration#result

[1.9.1](https://github.com/ruby/ruby/blob/1306ae8a461b30fbc59aedea7262958cc5775137/enumerator.c#L792)
[1.9.2](https://github.com/ruby/ruby/blob/e80d0a9631cdfebc53c1a111eb2162fe3f1dc67c/enumerator.c#L1097-L1098)
Implement StopIteration#result
Merge pull request #2477 from kachick/implement-stopiteration-result

Implement StopIteration#result