bug with String.contains polyfill

[donal-keane]

Using and loving this library. Found a bug with the String.contains polyfill. Another library was calling a for in loop on a string and the contains function was showing as a property. That library was then calling the function anonymously which led to an exception being thrown. We included a fix in our code before importing your library and I'm sharing it with you if you are interested in applying it to this library:
if(!String.prototype.contains){
Object.defineProperty(String.prototype, 'contains', {
enumerable: false,
value: function() {
return String.prototype.indexOf.apply(this, arguments) !== -1;
}
});
}

You can test out the fix by running the following code before and after this fix:
for(var i in ''){ console.log(i);var fn = ''[i]; fn();}