modify isSafeInteger polyfill

[DiligentYe]

Number.MAX_SAFE_INTEGER is defined by ES6, so Number.MAX_SAFE_INTEGER should be replaced by Math.pow(2, 53) - 1 there.

[legodude17]

Don't you mean isSafeInteger and not isSaftInteger?

[DiligentYe]

Oh, sorry! I mean isSafeInteger!

[legodude17]

That makes much more sense. 😄

[getify]

The assumption (though not stated) is that this polyfill would have relied on Number.MAX_SAFE_INTEGER having been defined, too. I think most usages would have done that, so I think it's safe. Perhaps at most we just need a code comment mentioning that assumption, rather than duplicating Math.pow(..) logic.