Fix clear() to also reset the subtype of a binary value

clear() on a binary value cleared the bytes but left the subtype
untouched, so the result was not equal to a default-constructed
binary value even though the documentation says clear() has the same
effect as *this = basic_json(type()). The fix calls
byte_container_with_subtype::clear_subtype() alongside the existing
clear() call.

Extended the "filled binary" clear() test in unit-modifiers.cpp with
a case that uses a subtype, since the existing cases only covered
binary values without one.

Fixes #5669.

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

docs/mkdocs/docs/api/basic_json/clear.md

@@ -7,15 +7,15 @@ void clear() noexcept;
 Clears the content of a JSON value and resets it to the default value as if [`basic_json(value_t)`](basic_json.md) would
 have been called with the current value type from [`type()`](type.md):
 
-| Value type | initial value        |
-|------------|----------------------|
-| null       | `null`               |
-| boolean    | `false`              |
-| string     | `""`                 |
-| number     | `0`                  |
-| binary     | An empty byte vector |
-| object     | `{}`                 |
-| array      | `[]`                 |
+| Value type | initial value                          |
+|------------|-----------------------------------------|
+| null       | `null`                                  |
+| boolean    | `false`                                 |
+| string     | `""`                                    |
+| number     | `0`                                     |
+| binary     | An empty byte vector with no subtype    |
+| object     | `{}`                                    |
+| array      | `[]`                                    |
 
 Has the same effect as calling
 
@@ -56,3 +56,4 @@ All iterators, pointers, and references related to this container are invalidate
 
 - Added in version 1.0.0.
 - Added support for binary types in version 3.8.0.
+- Fixed in version 3.13.0 to also clear the subtype of a binary value; before, the subtype was left unchanged.