Add header to async calls

Author: chinandrew

src/client/delphi_epidata.py

@@ -728,7 +728,7 @@ async def async_make_calls(param_combos):
       """Helper function to asynchronously make and aggregate Epidata GET requests."""
       tasks = []
       connector = TCPConnector(limit=batch_size)
-      async with ClientSession(connector=connector) as session:
+      async with ClientSession(connector=connector, headers=_HEADERS) as session:
         for param in param_combos:
           task = asyncio.ensure_future(async_get(param, session))
           tasks.append(task)