Merge pull request #8 from andrepew/master

When an Android app doesn't have the INTERNET permission, getHeaderFields() returns null resulting in NPE

src/main/java/com/github/nkzawa/engineio/client/transports/PollingXHR.java

@@ -201,10 +201,15 @@ public class PollingXHR extends Polling {
                         }
 
                         Map<String, String> headers = new TreeMap<String, String>(String.CASE_INSENSITIVE_ORDER);
-                        for (String key : xhr.getHeaderFields().keySet()) {
+
+                        Map<String, List<String>> xhrHeaderFields = xhr.getHeaderFields();
+                        if(xhrHeaderFields != null) {
+                            for (String key : xhrHeaderFields.keySet()) {
                                 if (key == null) continue;
                                 headers.put(key, xhr.getHeaderField(key));
                             }
+                        }
+
                         self.onResponseHeaders(headers);
 
                         final int statusCode = xhr.getResponseCode();