fixed races conditions found in server_test.go

This pull request aims at fixing a race race condition error as reported by the GO race detector:

==================
{"timestamp":1390798621.757039070,"process_id":9430,"source":"router.global","log_level":"info","message":"State is stale; NOT pruning","data":null}
Found 12 data race(s)
FAIL github.com/cloudfoundry/gorouter/router 14.450s
==================
WARNING: DATA RACE
Write by goroutine 104:
  github.com/cloudfoundry/gorouter/server_test.countReader.Read()
      /Users/jackfeng/go/src/github.com/cloudfoundry/gorouter/server/server_test.go:982 +0xa0
  github.com/cloudfoundry/gorouter/server_test.(*countReader).Read()
      /Users/jackfeng/go/src/github.com/cloudfoundry/gorouter/server/server_test.go:0 +0xec
  io.(*LimitedReader).Read()
      /usr/local/go/src/pkg/io/io.go:394 +0x13f
  io/ioutil.(*nopCloser).Read()
      /usr/local/go/src/pkg/io/ioutil/blackhole.go:1 +0x7d
  io.(*multiReader).Read()
      /usr/local/go/src/pkg/io/multi.go:13 +0xe0
  io.Copy()
      /usr/local/go/src/pkg/io/io.go:348 +0x1d8
  net/http.(*transferWriter).WriteBody()
      /usr/local/go/src/pkg/net/http/transfer.go:189 +0x225
  net/http.(*Request).write()
      /usr/local/go/src/pkg/net/http/request.go:387 +0xa10
  net/http.(*persistConn).writeLoop()
      /usr/local/go/src/pkg/net/http/transport.go:780 +0x20e
  gosched0()
      /usr/local/go/src/pkg/runtime/proc.c:1218 +0x9f

Previous read by goroutine 48:
  github.com/cloudfoundry/gorouter/server_test.TestRequestBodyLimit()
      /Users/jackfeng/go/src/github.com/cloudfoundry/gorouter/server/server_test.go:1014 +0x306
  testing.tRunner()
      /usr/local/go/src/pkg/testing/testing.go:353 +0x12f
  gosched0()
      /usr/local/go/src/pkg/runtime/proc.c:1218 +0x9f