[osx] Fix loop closing with pending requests

The main loop will now wait for all the requests to
complete before closing.

This will make the FileSource on OSX block longer than
it should at shutdown.

platform/darwin/http_request_nsurl.mm

@@ -5,6 +5,7 @@
 #include <mbgl/util/async_task.hpp>
 #include <mbgl/util/time.hpp>
 #include <mbgl/util/parsedate.h>
+#include <mbgl/util/run_loop.hpp>
 
 #import <Foundation/Foundation.h>
 
@@ -96,6 +97,11 @@ HTTPNSURLRequest::HTTPNSURLRequest(HTTPNSURLContext* context_,
       context(context_),
       existingResponse(existingResponse_),
       async([this] { handleResponse(); }) {
+    // Hold the main loop alive until the request returns. This
+    // is needed because completion handler runs in another
+    // thread and will notify this thread using AsyncTask.
+    util::RunLoop::Get()->ref();
+
     @autoreleasepool {
         NSURL* url = [NSURL URLWithString:@(url_.c_str())];
         if (context->accountType == 0 &&
@@ -129,6 +135,7 @@ HTTPNSURLRequest::HTTPNSURLRequest(HTTPNSURLContext* context_,
 }
 
 HTTPNSURLRequest::~HTTPNSURLRequest() {
+    util::RunLoop::Get()->unref();
     assert(!task);
 }