lunr.Index.prototype.idf() seems to fail when the term is an all-lowercase property of Object.prototype.

[gitgrimbo]

For example, an index with "constructor" or "UNDERUNDERprotoUNDERUNDER" terms in it will cause idf() to return the Object.prototype property of the same name.

(UNDER is _. Used here to avoid GFM stripping the _s and turning the property bold)

Because the returned property of Object.prototype is not a number, the subsequent calculations return NaN.

Pull request with tests to follow.

[olivernn]

I believe this is now fixed

Please re-open if you continue to see any problems.