Remove unnecessary bits and make String#startsWith and String#endsWith conform to spec
startsWith/endsWith: Add tests for null and undefined this values.
Redo startsWith/endsWith tests
simplify TypeError throwing for startsWith & endsWith
fix typo
Use slice instead of indexOf for startsWith & endsWith
Merge pull request #105 from Daniel-Hug/patch-5

String#startsWith / String#endsWith changes