Fix stringifier checks, see Mozilla bug 798151
Fix stringifier checks, see Mozilla bug 798151

ayg/testharness.js,hallvors/testharness.js,jyasskin/testharness.js,zqzhang/testharness.js,jyasskin/testharness.js,hallvors/testharness.js,bboyle/testharness.js,RatneshThakur/testharness.js