Fix browserification

Browserify fails to resolve the "./http-proxy/" as "./http-proxy/index.js" but as "./http-proxy.js" (so nothing works)
Beeing explicit here does not cost much for http-proxy, yet it's intrinsically complicated for browserify to fix (as trailing slash might be used as a pollyfill shim for native/non-natives addons i.e.  require('url/') vs require('url') )

lib/http-proxy.js

@@ -1,7 +1,7 @@
 var http = require('http'),
  https = require('https'),
  url = require('url'),
- httpProxy = require('./http-proxy/');
+ httpProxy = require('./http-proxy/index.js');
 
 /**
  * Export the proxy "Server" as the main export.