fix(Observer): anonymous observers now allow missing handlers

Errors were being thrown in situations where a handler was not present on a partial observer

fixes #723

src/Observer.ts

@@ -1,6 +1,6 @@
 export interface Observer<T> {
-  next(value: T): void;
-  error(err?: any): void;
-  complete(): void;
-  isUnsubscribed: boolean;
+  next?: (value: T) => void;
+  error?: (err?: any) => void;
+  complete?: () => void;
+  isUnsubscribed?: boolean;
 }

src/Subscriber.ts

@@ -83,15 +83,24 @@ export class Subscriber<T> extends Subscription<T> implements Observer<T> {
   }
 
   _next(value: T): void {
-    this.destination.next(value);
+    const destination = this.destination;
+    if (destination.next) {
+      destination.next(value);
+    }
   }
 
   _error(err: any): void {
-    this.destination.error(err);
+    const destination = this.destination;
+    if (destination.error) {
+      destination.error(err);
+    }
   }
 
   _complete(): void {
-    this.destination.complete();
+    const destination = this.destination;
+    if (destination.complete) {
+      destination.complete();
+    }
   }
 
   next(value?: T): void {