This patch does a step towards Fetch API:
- implements Request object to some extend. The Request object will be
sent in RequestWillBeSent event.
- implements InterceptedRequest which extends from Request and allows
for request modification. The InterceptedRequest does not
conform to Fetch API spec - there seems to be nothing related to
amending in-flight request.
- adds test to make sure that request can change headers.
References #26
7b59a89695