toString function is override, toString(undefined) return [object Window], not as expect

[missjing]

a few months ago, we use the polyfill service to generate a resource for Map&Set: https://cdn.polyfill.io/v2/polyfill.js?features=Map,Set&flags=always,gated. In order to solve the 'Map/Set is not defined' problems on old browsers, like Android 4.4

but, recently, we met a new problem, something not act as expect
in ios 8.1, we found the 'toString' function act different as other new ios versions
when we loaded the polyfill resource above, we found the toString was override
https://github.com/Financial-Times/polyfill-service/blob/master/packages/polyfill-library/polyfills/Symbol/polyfill.js#L215

toString(undefined) return [object Window], not [object undefined], this result caused other problem in our project, so we wish u can solve this problem.

finally, simulate the problem code line

[JakeChampion]

I think this would be solved by making the function run in strict mode because in strict mode an undefined context/this does not get changed to the global object.

We would change:
https://github.com/Financial-Times/polyfill-library/blob/master/polyfills/Symbol/polyfill.js#L215-L218

to this:

descriptor.value = function () {
	'use strict';
	var str = toString.call(this);
	return (str === '[object String]' && onlySymbols(this)) ? '[object Symbol]' : str;
};

[JakeChampion]

This was fixed in #193