feat(request): add request.initiator() method.

[atersolis]

add request.initiator() method, with tests and api doc

Requested by #1395

I was looking for this feature, and after reading the code, I realised it was really easy to implement.
All it took was 8 lines (+doc and tests). I wonder why it was not added earlier ?

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[atersolis]

Oh my bad, I read the issue too fast, I didn't see #1817
Well, there were no tests, so I guess it's not completely useless
It's been more than a year since #1817 though, so what's going on ?

[jschfflr]

This change looks good to me!

[jschfflr]

@atersolis Could you make sure to sign the CLA and let the @googlebot know? Happy to land your change then :)

[jschfflr]

As there hasn't been an update on this change for a while, closing this in favor of #7614