Check that an argument refers to an object, not a primitive,
before invoking WeakMap.get().

Fixes #16.

Tests run (and passed) under node v0.10 with --harmony:
  test/testHandlers.js
  test/testReflect.js
  test/testRegression.js
Merge pull request #17 from metamatt/issue16

Fix #16: don't pass primitives to WeakMap.get