fix generator test

[alsotang]

now, https://github.com/tj/co/blob/e679907d5e06b48e5d7523a856b75ca21eec8349/test/generators.js is the same as https://github.com/tj/co/blob/e679907d5e06b48e5d7523a856b75ca21eec8349/test/generator-functions.js .

work() would produce a generator, while work is a generator function. so I think we should explicit call work, avoiding it enter this condition: https://github.com/tj/co/blob/e679907d5e06b48e5d7523a856b75ca21eec8349/index.js#L51

then the test make sense

[alsotang]

why ignore this pr?

do you find https://github.com/tj/co/blob/master/test/generator-functions.js and https://github.com/tj/co/blob/master/test/generators.js are the same?