fix(imitate): fix issue #66 with imitate()

Fix imitate() so that existing listeners of a mimic stream are transferred to the imitation target
once imitate() is called.

Closes issue #66.

src/core.ts

@@ -1718,6 +1718,10 @@ export class Stream<T> implements InternalListener<T> {
       'https://github.com/staltz/xstream#faq');
     }
     this._target = target;
+    for (let ils = this._ils, N = ils.length, i = 0; i < N; i++) {
+      target._add(ils[i]);
+    }
+    this._ils = [];
   }
 
   /**

tests/operator/imitate.ts

@@ -266,4 +266,22 @@ describe('Stream.prototype.imitate', () => {
     });
     done();
   });
+
+  it('should transfer existing listeners to imitation target', (done) => {
+    const mimic = xs.create<number>();
+    const expected = [0, 1, 2];
+
+    mimic.addListener({
+      next: (x: number) => {
+        assert.equal(x, expected.shift());
+      },
+      error: (err: any) => done(err),
+      complete: () => {
+        assert.equal(expected.length, 0);
+        done();
+      },
+    });
+
+    mimic.imitate(xs.periodic(50).take(3));
+  });
 });