Accepting custom fetch in ApolloBoost

[mbaranovski]

Accepting custom fetch to be passed when creating ApolloClient instance. It solves #3578

Checklist:

• Make sure all of the significant new logic is covered by tests

[apollo-cla]

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[mbaranovski]

Build on CircleCI timed out. Needs to be retriggered I believe.

[hwillson]

This looks great @mbaranovski - thanks very much! We're all set here - merging shortly.

[mbaranovski]

Thanks, @hwillson. I've just noticed that my test is a false positive. No assertions are being triggered. It is a simple change. I've made a commit. Do you want me to open a separate PR for that or you will add it in some other PR? mbaranovski@eeb142e

[hwillson]

No problem @mbaranovski - if you could open that in a separate PR, that would be great. Thanks!