Overload take while to accept a type guard

[chrisguttandin]

Description:

This pull request overloads the takeWhile operator with an additional signature to accept a type guard.

I tried to keep the code as similar as possible to the corresponding overload signature of the filter operator. Please let me know, if I should change something.

I wasn't sure if the JSDoc should be changed as well. The return type of the filter operator is just Observable (without a generic) whereas the one of the takeWhile operator is Observable<T>.

Related issue (if exists):

This PR fixes #4081.

[cartant]

Thanks. Looks good.

Could I get you to add some dtslint tests, too? For testing typings, they're much less cumbersome. And I'd like typings changes to be accompanied by dtslint tests so that we can build out coverage of the typings.

Have a look at the find tests for examples:

https://github.com/ReactiveX/rxjs/blob/master/spec-dtslint/operators/find-spec.ts

To run the tests, use this command:

npm run dtslint

[chrisguttandin]

Hi, @cartant I added two dtslint tests. I never wrote a dtslint test before, so please let me know if I should change something.

[cartant]

LTGM.

[benlesh]

Thanks, @chrisguttandin!