fix(bindCallback): properly set context of input function

[mpodlasin]

Current behaviour:
Currently when input function (function that is passed as argument to bindCallback) is internally called, its context is set to context of underlying observable. If input function is not bound beforehand it is possible to implement:

function intercept(cb) {
   const observable = this;
   // do something with underlying observable
}
const boundIntercept = Observable.bindCallback(intercept);
boundIntercept(); // intercept is called with underlying observable as this

Expected behaviour
Since output function (function returned by `bindCallback) should behave identically as input function (apart from changing callback to observable) it is reasonable to assume that context of ouput function will become context of input function.

const boundMethod = Observable.bindCallback(someObject.method);
boundMethod.call(someObject) // calls method with someObject as this (as it was probably intended)

In fact this is what rxjs 4 does, as seen here:
https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/perf/operators/fromcallback.js#L41

This pr implements such behaviour.

Related
This pr does not change operator api. There is still no option to manually set context, which is possible in rxjs 4:

const bound = Observable.fromCallback(func, context);
bound() // func is called with context as this

Please let me know if I should add such functionality in this pr, create another pr or drop context setting functionality altogether.

Have a great day!

[coveralls]

Coverage increased (+0.0003%) to 97.689% when pulling bcd5b93 on Podlas29:bind-callback-this into faabd3e on ReactiveX:master.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.