es6 feature in middleware

[lhtdesignde]

Hi, I'm having a problem serving my webpack bundle to older browsers. I'm using your library but because of one little "const" in middleware.js it breaks.

fetch-wrap/src/middleware.js

Line 150 in 91faed9

const reviver = (receiveOptions && typeof receiveOptions.reviver !== undefined)

Would it be possible to replace "const" here with var?

[lhtdesignde]

Thanks for merging it in!