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') )

Author: 131

Diff for: 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.