fix JSON stringify to escape special characters

When converting from non string types the converted string was not escaped

Here is an example / testcase that was fixed by this PR.

```
template <typename T>
void PrintJSON(T&& v)
{
    std::stringstream ss;
    Poco::JSON::Array json;
    json.add(std::forward<T>(v));
    json.stringify(ss);
    std::cout << ss.str() << std::endl;
}

int main()
{
    std::string teststr = "hello \\ world";
    Poco::Dynamic::Array testvec{ teststr };

    PrintJSON(teststr);
    PrintJSON(testvec);
}
```

The code generated the following output. Notice that the second string is not valid JSON, because of the not escaped backslash.

```
["hello \\ world"]
[[ "hello \ world" ]]
```