fix: handle responses without content type (#101)

This fixes NullPointerException when response doesn't have content type. Exception stack trace:

```
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String okhttp3.MediaType.toString()' on a null object reference
        at io.socket.engineio.client.transports.PollingXHR$Request.onLoad(PollingXHR.java:271)
        at io.socket.engineio.client.transports.PollingXHR$Request.access$700(PollingXHR.java:148)
        at io.socket.engineio.client.transports.PollingXHR$Request$1.onResponse(PollingXHR.java:232)
        at okhttp3.RealCall$AsyncCall.execute(RealCall.java:141)
        at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1162)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:636)
        at java.lang.Thread.run(Thread.java:764)
```

src/main/java/io/socket/engineio/client/transports/PollingXHR.java

@@ -268,10 +268,10 @@ private void onResponseHeaders(Map<String, List<String>> headers) {
 
         private void onLoad() {
             ResponseBody body = response.body();
-            String contentType = body.contentType().toString();
+            MediaType mediaType = body.contentType();
 
             try {
-                if (BINARY_CONTENT_TYPE.equalsIgnoreCase(contentType)) {
+                if (mediaType != null && BINARY_CONTENT_TYPE.equalsIgnoreCase(mediaType.toString())) {
                     this.onData(body.bytes());
                 } else {
                     this.onData(body.string());