stringify the error object in header

lib/requestHandler.js

@@ -44,7 +44,7 @@ const getErrorResponse = (error, fullUrl) => {
     header: {
       'Content-Type': 'text/html; charset=utf-8',
       'Proxy-Error': true,
-      'Proxy-Error-Message': error || 'null'
+      'Proxy-Error-Message': error ? JSON.stringify(error) : 'null'
     },
     body: requestErrorHandler.getErrorContent(error, fullUrl)
   };
@@ -116,7 +116,7 @@ function fetchRemoteResponse(protocol, options, reqData, config) {
 
             // set origin content length into header
             resHeader['x-anyproxy-origin-content-length'] = originContentLen;
-            
+
             // only do unzip when there is res data
             if (ifServerGzipped && originContentLen) {
               refactContentEncoding();