IE8 issue when wysihtml5 tried to call getAttibute on a boolean

[This line](https://github.com/Edicy/wysihtml5/commit/b43ab939e5919c87035c88664b1ab7372b12d7c4#diff-da34feb2e61308baec3ec93125e6b91cL8124) calls `show` with a parameter that may be a boolean. This PR fixes the callee to deal with a potential boolean. Before that patch, it tried to call getAttribute on a boolean object with obviously fails.