fix regression from jest 17.0.1 mutating options.extensions (#2089)
fix regression from jest 17.0.1 mutating options.extensions (#2089)


ide/jest,bookman25/jest,cpojer/jest,skovhus/jest,facebook/jest,cpojer/jest,mpontus/jest,ide/jest,mpontus/jest,skovhus/jest,mpontus/jest,bookman25/jest,ide/jest,bookman25/jest,facebook/jest,facebook/jest,skovhus/jest,cpojer/jest