fix(subscribe): allows functions where bind has been patched to be weird

Apparently, code exists in the wild that will patch function bind to do something other than return a function that will execute the function instance, so we cannot rely on bind.

Resolves ReactiveX#6783

spec/Observable-spec.ts

@@ -191,6 +191,34 @@ describe('Observable', () => {
   });
 
   describe('subscribe', () => {
+    it('should work with handlers with hacked bind methods', () => {
+      const source = of('Hi');
+      const results: any[] = [];
+      const next = function (value: string) {
+        results.push(value);
+      }
+      next.bind = () => { /* lol */};
+
+      const complete = function () {
+        results.push('done');
+      }
+      complete.bind = () => { /* lol */};
+
+      source.subscribe({ next, complete });
+      expect(results).to.deep.equal(['Hi', 'done']);
+    });
+
+    it('should work with handlers with hacked bind methods, in the error case', () => {
+      const source = throwError(() => 'an error');
+      const results: any[] = [];
+      const error = function (value: string) {
+        results.push(value);
+      }
+
+      source.subscribe({ error });
+      expect(results).to.deep.equal(['an error']);
+    });
+
     it('should be synchronous', () => {
       let subscribed = false;
       let nexted: string;

src/internal/Subscriber.ts

@@ -166,9 +166,9 @@ export class SafeSubscriber<T> extends Subscriber<T> {
       } else {
         context = observerOrNext;
       }
-      next = next?.bind(context);
-      error = error?.bind(context);
-      complete = complete?.bind(context);
+      next = next && ((value: T) => context.next(value));
+      error = error && ((err: any) => context.error(err));
+      complete = complete && (() => context.complete());
     }
 
     // Once we set the destination, the superclass `Subscriber` will