fix(pathRewrite): path can be empty string (chimurai#110)

* allow empty string as path.
* disallow non-string path.

Author: sunnylqm

lib/index.js

@@ -119,7 +119,7 @@ function HttpProxyMiddleware(context, opts) {
         if (pathRewriter) {
             var path = pathRewriter(req.url, req);
 
-            if (path) {
+            if (typeof path === 'string') {
                 req.url =  path;
             } else {
                 logger.info('[HPM] pathRewrite: No rewritten path found. (%s)', req.url);