fix Future<const T>::value()

Summary: I'm still a little mystified how one makes a Future<const T> (I tried to make one explicitly in a test and failed), but this is clearly the bug @reedriley sees in https://phabricator.fb.com/D1620805#21

Test Plan:
fbconfig titan/cachius/test
fbmake

Reviewed By: reedriley@fb.com

Subscribers: trunkagent, net-systems@, fugalh, exa, njormrod, reedriley, folly-diffs@

FB internal diff: D1630378

Blame Revision: D1620805

folly/wangle/Future-inl.h

@@ -179,7 +179,7 @@ template <class T>
 typename std::add_lvalue_reference<const T>::type Future<T>::value() const {
   throwIfInvalid();
 
-  return core_->value();
+  return core_->getTry().value();
 }
 
 template <class T>