Make json_pointer::back const (resolves #1764)

test/src/unit-json_pointer.cpp

@@ -559,6 +559,10 @@ TEST_CASE("JSON pointers")
         CHECK(!ptr.empty());
         CHECK(j[ptr] == j["answer"]["everything"]);
 
+        // check access via const pointer
+        const auto cptr = ptr;
+        CHECK(cptr.back() == "everything");
+
         ptr.pop_back();
         ptr.pop_back();
         CHECK(ptr.empty());