testing: fix data race found while running tsuru tests
testing: fix data race found while running tsuru tests

==================
WARNING: DATA RACE
Read by goroutine 215:
  runtime.mapaccess2_faststr()
      /usr/local/Cellar/go/1.4.2/libexec/src/runtime/hashmap_fast.go:281 +0x0
  github.com/fsouza/go-dockerclient/testing.(*DockerServer).inspectImage()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:796 +0x12f
  github.com/fsouza/go-dockerclient/testing.*DockerServer.(github.com/fsouza/go-dockerclient/testing.inspectImage)·fm()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:117 +0x52
  github.com/fsouza/go-dockerclient/testing.func·001()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:268 +0x50c
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1265 +0x4e
  github.com/fsouza/go-dockerclient/vendor/github.com/gorilla/mux.(*Router).ServeHTTP()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/vendor/github.com/gorilla/mux/mux.go:98 +0x377
  github.com/fsouza/go-dockerclient/testing.(*DockerServer).ServeHTTP()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:229 +0x29b
  net/http.serverHandler.ServeHTTP()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1703 +0x1f6
  net/http.(*conn).serve()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1204 +0x1087

Previous write by goroutine 218:
  runtime.mapassign1()
      /usr/local/Cellar/go/1.4.2/libexec/src/runtime/hashmap.go:383 +0x0
  github.com/fsouza/go-dockerclient/testing.(*DockerServer).pullImage()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:721 +0x597
  github.com/fsouza/go-dockerclient/testing.*DockerServer.(github.com/fsouza/go-dockerclient/testing.pullImage)·fm()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:113 +0x52
  github.com/fsouza/go-dockerclient/testing.func·001()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:268 +0x50c
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1265 +0x4e
  github.com/fsouza/go-dockerclient/vendor/github.com/gorilla/mux.(*Router).ServeHTTP()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/vendor/github.com/gorilla/mux/mux.go:98 +0x377
  github.com/fsouza/go-dockerclient/testing.(*DockerServer).ServeHTTP()
      /Users/cezar.sa/go/src/github.com/fsouza/go-dockerclient/testing/server.go:229 +0x29b
  net/http.serverHandler.ServeHTTP()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1703 +0x1f6
  net/http.(*conn).serve()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1204 +0x1087

Goroutine 215 (running) created at:
  net/http.(*Server).Serve()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1751 +0x3cd
  net/http.Serve()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1606 +0xdc

Goroutine 218 (running) created at:
  net/http.(*Server).Serve()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1751 +0x3cd
  net/http.Serve()
      /usr/local/Cellar/go/1.4.2/libexec/src/net/http/server.go:1606 +0xdc
==================

eliasgs/go-dockerclient,fsouza/go-dockerclient,davidstack/go-dockerclient,csrwng/go-dockerclient,swagiaal/go-dockerclient,paulbellamy/go-dockerclient,samuelkarp/go-dockerclient,dcbw/go-dockerclient,achanda/go-dockerclient,tarsisazevedo/go-dockerclient,davidstack/go-dockerclient,styner32/go-dockerclient,giantswarm/go-dockerclient,ybogdanov/go-dockerclient,jefferai/go-dockerclient,w4ngyi/go-dockerclient,bradgignac/go-dockerclient,cloudimmunity/go-dockerclientx,styner32/go-dockerclient,lindenlab/go-dockerclient,jandre/go-dockerclient,slene/go-dockerclient,liron-l/go-dockerclient,orivej/go-dockerclient,codekitchen/go-dockerclient,freeformz/go-dockerclient,deepglint/go-dockerclient,daniel-garcia/go-dockerclient,bboreham/go-dockerclient,bfosberry/go-dockerclient,42wim/go-dockerclient,thomasv314/go-dockerclient,flowlo/go-dockerclient,ybogdanov/go-dockerclient,derekwaynecarr/go-dockerclient,csrwng/go-dockerclient,bfosberry/go-dockerclient,orivej/go-dockerclient,gambol99/go-dockerclient,dcbw/go-dockerclient-dcbw,tarsisazevedo/go-dockerclient