Make node-fetch tests contribute to coverage report

[ronag]

How do make node-fetch tests to be included in coverage report?

[ronag]

@szmarczak @mcollina any experience with this?

[szmarczak]

No experience, but I'll see and let you know in a few mins.

[szmarczak]

https://github.com/istanbuljs/nyc#combining-reports-from-multiple-runs

[szmarczak]

I'll send a PR