test(Subject): add tests for subjects used as Observer

Add tests for Subject, BehaviorSubject and ReplaySubject to check whether they can be used as plain
observers when given to Observable.subscribe().

Related to issue #825.

spec/subject-spec.js

@@ -471,4 +471,20 @@ describe('Subject', function () {
     expect(output).toEqual([1,2,3,4,5]);
     expect(outputComplete).toBe(true);
   });
+
+  it('should be an Observer which can be given to Observable.subscribe', function (done) {
+    var source = Observable.of(1, 2, 3, 4, 5);
+    var subject = new Subject();
+    var expected = [1, 2, 3, 4, 5];
+
+    subject.subscribe(
+      function (x) {
+        expect(x).toBe(expected.shift());
+      },
+      done.fail,
+      done
+    );
+
+    source.subscribe(subject);
+  });
 });

spec/subjects/behavior-subject-spec.js

@@ -3,6 +3,7 @@ var Rx = require('../../dist/cjs/Rx');
 
 var BehaviorSubject = Rx.BehaviorSubject;
 var nextTick = Rx.Scheduler.nextTick;
+var Observable = Rx.Observable;
 
 describe('BehaviorSubject', function () {
   it('should extend Subject', function (done) {
@@ -115,4 +116,20 @@ describe('BehaviorSubject', function () {
     )).toBe(sourceTemplate);
     expectObservable(subscriber1).toBe(expected1);
   });
+
+  it('should be an Observer which can be given to Observable.subscribe', function (done) {
+    var source = Observable.of(1, 2, 3, 4, 5);
+    var subject = new BehaviorSubject(0);
+    var expected = [0, 1, 2, 3, 4, 5];
+
+    subject.subscribe(
+      function (x) {
+        expect(x).toBe(expected.shift());
+      },
+      done.fail,
+      done
+    );
+
+    source.subscribe(subject);
+  });
 });

spec/subjects/replay-subject-spec.js

@@ -3,6 +3,7 @@ var Rx = require('../../dist/cjs/Rx');
 
 var ReplaySubject = Rx.ReplaySubject;
 var nextTick = Rx.Scheduler.nextTick;
+var Observable = Rx.Observable;
 
 describe('ReplaySubject', function () {
   it('should extend Subject', function (done) {
@@ -206,4 +207,20 @@ describe('ReplaySubject', function () {
       expectObservable(subscriber1).toBe(expected1);
     });
   });
+
+  it('should be an Observer which can be given to Observable.subscribe', function (done) {
+    var source = Observable.of(1, 2, 3, 4, 5);
+    var subject = new ReplaySubject(3);
+    var expected = [3, 4, 5];
+
+    source.subscribe(subject);
+
+    subject.subscribe(
+      function (x) {
+        expect(x).toBe(expected.shift());
+      },
+      done.fail,
+      done
+    );
+  });
 });