Fix transaction building with no provided change address

Found an error here where the instance variable is getting called instead of the method, so passing in a blank change address will blow up.  

The tests over the change_address method work, it just wasn't being invoked.  It doesn't look like there are any tests at all over that 'build' method, so I didn't bother writing one.  It would be pretty trivial to test that that method gets called.  I'm just not really sure about your approach to testing, etc.