support construction from other basic_json types

Before this patch, `basic_json` types with different template arguments
were treated as `CompatibleArrayType`. Which sometimes leads to recursive
calls and stack overflows.

This patch adds a constructor and a `get` overload to deal with
different `basic_json` types.

include/nlohmann/detail/meta.hpp

@@ -233,7 +233,7 @@ struct is_compatible_complete_type
 {
     static constexpr bool value =
         not std::is_base_of<std::istream, CompatibleCompleteType>::value and
-        not std::is_same<BasicJsonType, CompatibleCompleteType>::value and
+        not is_basic_json<CompatibleCompleteType>::value and
         not is_basic_json_nested_type<BasicJsonType, CompatibleCompleteType>::value and
         has_to_json<BasicJsonType, CompatibleCompleteType>::value;
 };