Add basic test coverage of toArrayBuffer function

test/helpers_test.js

@@ -27,4 +27,21 @@ describe("Helpers", function() {
           assert.equal(getString(b), "\x00\xff\x80");
       });
   });
+
+  describe("toArrayBuffer", function() {
+      it('returns undefined when passed undefined', function() {
+          assert.strictEqual(toArrayBuffer(undefined), undefined);
+      });
+      it('returns ArrayBuffer when passed ArrayBuffer', function() {
+          var StaticArrayBufferProto = new ArrayBuffer().__proto__;
+          var anArrayBuffer = new ArrayBuffer();
+          assert.strictEqual(toArrayBuffer(anArrayBuffer), anArrayBuffer);
+      });
+      it('throws an error when passed a non Stringable thing', function() {
+          var madeUpObject = function() {};
+          var notStringable = new madeUpObject();
+          assert.throw(function() { toArrayBuffer(notStringable) },
+                       Error, /Tried to convert a non-stringable thing/);
+      });
+  });
 });