fix(service): `$nextLang` should be not unset parallel loadings

resolves #647

All tests are fine, but I would like to have a review on that one. 

Until now: If parallel (asynchronous) loadings would be invoked (by `use()`), the prior ones` `$nextLang` has been overridden by the next ones which causes the success handler NOT invoking `useLanguage(requestedKey)` (because of condition `$nextLang == requestedKey` is not true anymore)

Now with this change: Regardless which or how many languages are currently loading, `useLanguage(requestedKey)` will be invoked on success; even when there is still a running load.

Is someone unhappy with this?  I do not see a problem right now.

/cc @angular-translate/core