Fix async version of decode_request

so thath it does not fail if no payload at all is provided in the request,
eg. for an endpoint which really is (or at least should be) a simple GET...

swh/core/api/asynchronous.py

@@ -22,7 +22,8 @@ def encode_data_server(data, **kwargs):
 def decode_request(request):
     content_type = request.headers.get('Content-Type')
     data = yield from request.read()
-
+    if not data:
+        return {}
     if content_type == 'application/x-msgpack':
         r = msgpack_loads(data)
     elif content_type == 'application/json':