Support read-only and write-only drivers, closes #24

Some drivers don't take in sinks, and some drivers don't return sources.
We definitely shouldn't explode in those cases.

src/restartable.js

@@ -119,15 +119,17 @@ export default function restartable (driver, opts = {}) {
   function restartableDriver (sink$) {
     let filteredSink$ = sink$;
 
-    if (pauseSinksWhileReplaying) {
+    if (sink$ && pauseSinksWhileReplaying) {
       filteredSink$ = sink$.filter(_ => !replaying);
     }
 
     const source = driver(filteredSink$);
 
     let returnValue;
 
-    if (typeof source.subscribe === 'function') {
+    if (source === undefined || source === null) {
+      return source;
+    } else if (typeof source.subscribe === 'function') {
       returnValue = recordObservableSource({streams, log}, source);
     } else {
       returnValue = wrapSource({streams, log}, source);

test/node/restartable-test.js

@@ -0,0 +1,23 @@
+/* globals describe, it, before, after */
+import assert from 'assert';
+import {restartable} from '../../src/restart';
+import {Observable} from 'rx';
+
+
+describe('restartable', () => {
+  it('handles write only drivers', (done) => {
+    const testDriver = () => {};
+
+    restartable(testDriver)(Observable.empty());
+
+    done();
+  });
+
+  it('handles read only drivers', (done) => {
+    const testDriver = () => Observable.empty();
+
+    restartable(testDriver)();
+
+    done();
+  });
+});