Write specs for String#b
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

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