Keep multimaps with enum keys as arrays of pairs

With JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS, is_enum_keyed_map also matched
std::multimap and std::unordered_multimap. Storing them as objects throws
type_error.318 as soon as a key occurs twice, which is the normal case for
a multimap, so such values could no longer be serialized at all once the
macro was enabled, although they are stored losslessly as arrays of
[key, value] pairs without it.

Exclude maps with non-unique keys from is_enum_keyed_map. They are
detected by insert(value_type) returning an iterator rather than a
pair<iterator, bool>. Map-like types without such an insert() are still
treated as before.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/macros/json_use_objects_for_enum_keyed_maps.md

@@ -35,6 +35,11 @@ The default value is `0` (disabled — existing behavior is preserved).
     {"completed": "bb", "stopped": "aa"}
     ```
 
+!!! note "Maps with non-unique keys"
+
+    Maps that allow duplicate keys, such as `std::multimap<E, T>` or `std::unordered_multimap<E, T>`, are not affected
+    by the macro and are still stored as arrays of `[key, value]` pairs, as an object cannot hold duplicate keys.
+
 !!! note "Reading"
 
     Reading is not affected by the macro: a map with enum keys can always be read from both an array of pairs and an