Bump gorouter

$ git shortlog a87b4fb9..16c3ecd2
Dmitriy Kalinin and Pieter Noordhuis (9):
      Define interface for Varz
      Add NullVarz
      Initial set of proxy tests
      Add test for response to request from load balancer
      Pass configuration struct to NewProxy
      Refactor and move tests for 404/502 to proxy_test.go
      Refactor test for X-Forwarded-For
      Initial stab at WebSocket support
      Refactor error handling

Change-Id: Ibaef2984729e9ffeea19236066771d5147fba372