Mitmproxy: Add --keep-host-header

[mhils]

Steps to reproduce the problem:

1. mitmdump -R https://httpbin.org/
2. curl localhost:8080/headers

This returns "Host": "httpbin.org" instead of "Host": "localhost", which was what the original curl request sent. We want to add a --keep-host-header to disable our automatic host header rewrite here. For a discussion of the rationale, see https://github.com/mitmproxy/mitmproxy/pull/2034#issuecomment-280770573.

Once implemented, we should change the dns_spoofing example to use that instead of doing the requestheaders dance. First, however, let's keep the PR small and only add the switch.

[marncz]

Hello, I'd like to work on that issue as my first contribution to the project

[krsoninikhil]

Hi @marncz,
I was working on this and about to submit a PR, please consider other issues.

[marncz]

@krsoninikhil no problem, thanks for letting me know :)

[WHK102]

Related https://github.com/mitmproxy/mitmproxy/issues/786