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>

include/nlohmann/json.hpp

@@ -3833,6 +3833,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             case value_t::binary:
             {
                 m_data.m_value.binary->clear();
+                m_data.m_value.binary->clear_subtype();
                 break;
             }