Add support for Headers object

[andreynovikov]

In some Node environments request.headers is not a dictionary, but a Headers object: https://developer.mozilla.org/en-US/docs/Web/API/Headers
It would be great if the library can handle it natively without the need to convert it to dictionary.

[faisalman]

Hi, thank you for your suggestion. I've just added it in d5edcba

[andreynovikov]

Works great, thank you!