Keep blank URL parameters

I'm using mitmproxy in a situation where blank URL parameters are required (similar to this [older, rejected PR](https://github.com/cortesi/mitmproxy/pull/41)).  

This change does the "dumbest thing that could possibly work":
  - Are you open to adding this, and
  - Assuming this should be configurable, any thoughts on how the option should be exposed? A command-line option seems like overkill.