Throw type_error.313 when an unflatten key is a prefix of an earlier key

With an object type that keeps the insertion order (ordered_json), a key
such as "/a" can come after "/a/b". The final assignment then replaced the
object built for "/a/b" without an error, so {"/a/b":2,"/a":1} gave
{"a":1}. json never saw this because std::map visits the shorter key first.

Throw type_error.313 if the target of the final assignment is no longer
null, as json does for the same members. A null value keeps the container,
so {"/a/b":2,"/a":null} gives {"a":{"b":2}} for both object types.

Fixes #6105.

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

docs/mkdocs/docs/api/basic_json/unflatten.md

@@ -26,7 +26,8 @@ The function can throw the following exceptions:
 - Throws [`type_error.314`](../../home/exceptions.md#jsonexceptiontype_error314) if value is not an object
 - Throws [`type_error.315`](../../home/exceptions.md#jsonexceptiontype_error315) if object values are not primitive
 - Throws [`type_error.313`](../../home/exceptions.md#jsonexceptiontype_error313) if a key (JSON pointer) leads to a
-  conflicting nesting; example: `"invalid value to unflatten"`
+  conflicting nesting, for example if a key with a value other than `#!json null` is a prefix of another key (as `/a`
+  and `/a/b`), in whichever order the keys come; example: `"invalid value to unflatten"`
 - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in a key begins
   with '0'; example: `"array index '01' must not begin with '0'"`
 - Throws [`parse_error.107`](../../home/exceptions.md#jsonexceptionparse_error107) if a key is not empty and does not
@@ -90,3 +91,6 @@ Apart from these three cases, for a JSON value `j`, the following is always true
 - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) instead of
   [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) for a one-character array index that is
   not a digit (e.g., `/x`) in version 3.13.0, as it already did for longer ones.
+- Throws [`type_error.313`](../../home/exceptions.md#jsonexceptiontype_error313) for a key that is a prefix of an
+  earlier key in version 3.13.0; before, an object type that keeps the insertion order (such as
+  [`ordered_json`](../ordered_json.md)) silently replaced the nested value.