Change Json map/vector conversions to invoke begin() directly (#110)

... instead of using key_type/value_type/mapped_type. Because this could
otherwise false-positive on things like std::optional, which has a value_type
member type, but is not a container.

Fixes #109.

json11.hpp

@@ -107,14 +107,14 @@ class Json final {
 
     // Implicit constructor: map-like objects (std::map, std::unordered_map, etc)
     template <class M, typename std::enable_if<
-        std::is_constructible<std::string, typename M::key_type>::value
-        && std::is_constructible<Json, typename M::mapped_type>::value,
+        std::is_constructible<std::string, decltype(std::declval<M>().begin()->first)>::value
+        && std::is_constructible<Json, decltype(std::declval<M>().begin()->second)>::value,
             int>::type = 0>
     Json(const M & m) : Json(object(m.begin(), m.end())) {}
 
     // Implicit constructor: vector-like objects (std::list, std::vector, std::set, etc)
     template <class V, typename std::enable_if<
-        std::is_constructible<Json, typename V::value_type>::value,
+        std::is_constructible<Json, decltype(*std::declval<V>().begin())>::value,
             int>::type = 0>
     Json(const V & v) : Json(array(v.begin(), v.end())) {}