Add tests for relative and absolute redirect with a proxy

[edgurgel]

Add absolute and relative redirect tests using a proxy

I'm getting this message for the relative:

127.0.0.1:64644: ProtocolException("Error in HTTP connection: HttpException('Invalid HTTP request form (expected: authority or absolute, got: relative)',)",)

in function hackney_integration_tests:'-relative_redirect_request_follow_with_proxy/0-fun-0-'/2 
**error:{assertEqual,[{module,hackney_integration_tests},
              {line,134},
              {expression,"StatusCode"},
              {expected,200},
              {value,400}]}
  output:<<"">>

I'm not mitmproxy master so please double check if I'm using it correctly :)

[benoitc]

:+1:

[benoitc]

@edgurgel this error means we are returning a relative url somewhere on redirect. Which shouldn't happen ...