[fetch] Content-Length on HEAD (#135)

https://fetch.spec.whatwg.org/#http-network-or-cache-fetch

> If httpRequest's body is null and httpRequest's method is `HEAD`, `POST`, or `PUT`, set contentLengthValue to `0`.

Why is C-L set on HEAD requests?

See:
  http://httpwg.github.io/specs/rfc7230.html#message.body.length

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https://github.com/whatwg/fetch/issues/135