Fix handling of types with deleted rvalue conversion to string (#1421)

include/fmt/core.h

@@ -923,7 +923,7 @@ template <typename Context> struct arg_mapper {
 // A type constant after applying arg_mapper<Context>.
 template <typename T, typename Context>
 using mapped_type_constant =
-    type_constant<decltype(arg_mapper<Context>().map(std::declval<T>())),
+    type_constant<decltype(arg_mapper<Context>().map(std::declval<const T&>())),
                   typename Context::char_type>;
 
 enum { packed_arg_bits = 5 };