merge_or_update behavior differs based on selected overload #755

Describe the bug
The function merge_or_update in sorted_json_object.hpp overrides the wrong fields in the overload that takes rvalue reference. The overload that takes a const lvalue ref behaves as expected.
Source code link for latest commit to the file at the time of opening this bug:

The bug is that in the sorted_json_object&& overload, pos is searched using std::lower_bound, which returns the first element that should not appear before (*it).key(), but it does not check that it is actually that key before updating it.
For example, if I have a json object { "a": 5, "c": 19 } and I call merge_or_update with {"b":2} passed in a rvalue ref, the key b is not added to the json, rather the value of c changes to 2.
Again, the const sorted_json_object& behaves correctly.

merge_or_update should override the value of the correct key.

Passing an rvalue ref to merge_or_update removes existing keys if they don't exist in the input json.

Enumerate the steps to reproduce the bug
shown in example code

Include a small, self-contained example if possible

    const jsoncons::json source = jsoncons::json::parse(R"({"b":2})");

    jsoncons::json constRefTarget = jsoncons::json::parse(R"({"a":1,"c":3})");
    constRefTarget.merge_or_update(source);

    jsoncons::json rvalueTarget = jsoncons::json::parse(R"({"a":1,"c":3})");
    rvalueTarget.merge_or_update(jsoncons::json(source));

    std::cout << "expected:  {\"a\":1,\"b\":2,\"c\":3}\n";
    std::cout << "const&:    " << constRefTarget << "\n";
    std::cout << "&&:        " << rvalueTarget << "\n";

Output:

expected:  {"a":1,"b":2,"c":3}
const&:    {"a":1,"b":2,"c":3}
&&:        {"a":1,"c":2}

What compiler, architecture, and operating system?
Windows 11, x64, Visual Studio 18.10.1 build tools 14.51

• Compiler: MSVC
• Architecture (e.g. x86, x64) x64
• Operating system: Windows

What jsoncons library version?

• Latest release 1.10.0
• Other release All releases from 0.99.8
• master