Use fetch-ponyfill instead of isomorphic-fetch polyfill

[ctavan]

Following my reasoning in apollographql/apollo-client#2008 I think it would be a good practice if a library like apollo-fetch did not modify the global namespace by polyfilling fetch.

Is there any particular reason not to use https://github.com/qubyte/fetch-ponyfill instead? If you would accept a ponyfill instead of a polyfill I'd be happy to contribute a PR.

Bundle-size wise there should be no signifcant difference.

[giautm]

Link issues i facing on react-native :(

matthew-andrews/isomorphic-fetch#125

[joarwilk]

This really needs to be resolved. @ctavan is your PR ready?

[ctavan]

@joarwilk as outlined in #30 I believe apollo-fetch should not be bundled with a fetch ponyfill but leave that choice up to the user. I have prepared a PR (#30) for an apollo-fetch-polyfill package that comes with the cross-fetch polyfill. Still awaiting feedback from the maintainers though.

With that change it would be very simply to provide your own fetch-ponyfill when using the regular apollo-fetch which would then come without any polyfill.

[evans]

#71 uses cross-fetch, so this should be resolved