Non-overridable options in network layer middleware

[rricard]

Hello!

I'm trying to implement file upload via a middleware. This would work if I was able to override the request's options completely. However, this is currently not possible since the network layer will reapply its own options on top of what I defined (see https://github.com/apollostack/apollo-client/blob/aeefad5/src/networkInterface.ts#L186).

I'm creating a PR right away but if you wish to discuss this, we can do it here!