Fix clear() to also reset the subtype of a binary value (#5680)

* 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>

* Fix the table alignment in clear.md

Addresses review comment by @gregmarr.

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

---------

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

tests/src/unit-modifiers.cpp

@@ -155,6 +155,18 @@ TEST_CASE("modifiers")
                 CHECK(j == json(json::value_t::binary));
                 CHECK(j == json(k.type()));
             }
+
+            SECTION("filled binary with subtype")
+            {
+                json j = json::binary({1, 2, 3, 4, 5}, 42);
+                json const k = j;
+
+                j.clear();
+                CHECK(!j.empty());
+                CHECK(!j.get_binary().has_subtype());
+                CHECK(j == json(json::value_t::binary));
+                CHECK(j == json(k.type()));
+            }
         }
 
         SECTION("number (integer)")