default follow to true if not boolean

the previous behaviour was that `nofollow` defaulted to `false` if `nofollow` was not a boolean.

The new behaviour is that `nofollow` defaults to `!falsey` if it was not a boolean. This is a breaking change.

We now treat non boolean values as `nofollow = false`

This breaks `npm@>=1.4.16` with registries that return 301s and with internal corporate proxies