Don't freeze regexps (#65)

lib/freeze.js

@@ -1,5 +1,6 @@
 function isFreezable(object) {
   if (object === null) return false;
+  if (object instanceof RegExp) return false;
 
   return Array.isArray(object) ||
     typeof object === 'object';

test/freeze-spec.js

@@ -35,6 +35,13 @@ describe('u.freeze', () => {
     expect(Object.isFrozen(object.foo)).to.be.false;
   });
 
+  it('ignores regexps', () => {
+    const object = { foo: /\d/ };
+    u.freeze(object);
+
+    expect(Object.isFrozen(object.foo)).to.be.false;
+  });
+
   it('does not freeze children if the parent is already frozen', () => {
     const object = { foo: {} };
     Object.freeze(object);