google-api-go-generator: always reference the strings package
google-api-go-generator: always reference the strings package

Fixes generation after 49979a5f144 which stopped always referencing
strings.Replace and strings.Map.

R=campoy, campoy
CC=golang-dev
https://codereview.appspot.com/9706044

aquarat/google-api-go-client,Shino1024/google-api-go-client,abujehad139/google-api-go-client,maskofG/google-api-go-client,tmaiaroto/google-api-go-client,Shino1024/google-api-go-client,maskofG/google-api-go-client,aquarat/google-api-go-client,abujehad139/google-api-go-client,freehaha/freehaha-gapi,freehaha/freehaha-gapi,tmaiaroto/google-api-go-client