Write specs for String#b

* https://github.com/rubinius/rubinius/pull/2501#pullrequestreviewcomment-5416650
* https://github.com/rubinius/rubinius/pull/2501#pullrequestreviewcomment-5416641
* https://github.com/rubinius/rubinius/pull/2501#pullrequestreviewcomment-5431863
Implement String#b
Merge pull request #2501 from kachick/2.0-string-b

Implement String#b for 2.0