fix(policy): remove sealing to allow extension of policy methods

Since we returned a sealed object from the PolicyBase function, we
could not actually extend the `parseAddress` and `parseLinkAddress`
methods. Removing that line fixes it, and two tests are included

lib/policies/policy.js

@@ -103,7 +103,7 @@ function Policy(overrides) {
     },
   });
 
-  return Object.seal(this);
+  return this;
 }
 
 /**

test/unit/policies/policy.test.js

@@ -43,6 +43,20 @@ describe('Default Policy', function() {
         });
     });
 
+    it('should allow redefinition of parseAddress', function() {
+      var policy = amqp.Policy.merge({}, amqp.Policy.DefaultPolicy);
+      policy.parseAddress = function() { return { name: 'llamas' }; };
+      var addr = policy.parseAddress('testing');
+      expect(addr).to.eql({ name: 'llamas' });
+    });
+
+    it('should allow redefinition of parseLinkAddress', function() {
+      var policy = amqp.Policy.merge({}, amqp.Policy.DefaultPolicy);
+      policy.parseLinkAddress = function() { return { name: 'llamas' }; };
+      var addr = policy.parseLinkAddress('testing');
+      expect(addr).to.eql({ name: 'llamas' });
+    });
+
     it('should not add a SASL layer for anonymous auth by default', function() {
       var policy = amqp.Policy.merge({
         connect: {