Add default template param for map key in MapUtil

Summary:
I've got this codemod that is turning `stl::mapGetDefault` -> `folly::get_default`, and noticed one of the generated diffs failed to compile: D66440835

Compiler couldn't infer the `{p1, p2}` key as a pair type, and noticed that we just need to give it a bit of a hint.

Reviewed By: Gownta

Differential Revision: D66450949

fbshipit-source-id: 7e0f1487a3954eda6574c3bc4d4b1d0cb0ac7bb1

folly/container/MapUtil.h

@@ -37,7 +37,7 @@ namespace folly {
  * Given a map and a key, return the value corresponding to the key in the map,
  * or a given default value if the key doesn't exist in the map.
  */
-template <typename Map, typename Key>
+template <typename Map, typename Key = typename Map::key_type>
 typename Map::mapped_type get_default(const Map& map, const Key& key) {
   auto pos = map.find(key);
   return (pos != map.end()) ? (pos->second) : (typename Map::mapped_type{});