Update proxy.js

Using the following test_proxy.js:

var proxy = require('redbird')({port: 8080});

proxy.register("localhost:8080/test_1", "http://localhost:1337");
proxy.register("localhost:8080/test_2", "http://localhost:1338");
proxy.register("localhost:8080/test_3", "http://localhost:1339");

If I attempted to use the url "http://localhost:8080/test_unknown" the node server would throw the following error"

      if(target.pathname){
               ^
      TypeError: Cannot read property 'pathname' of undefined

The target variable is undefined because none of the routes/keys could be matched and the ".pathname" dereference causes the process to terminate.

I've just added a test make sure its valid and then log a warning about the target, and also altered the other warning to indicate a host which wasn't found.

Tested on OSX 10.9.