Update test_boost_json.cpp

[beached]

This shows the Boost JSON in a better light and uses safer at( ) instead of operator[] that will insert if the member doesn't exist. At throws. It also uses a more idiomatic, for read only JSON, use of memory_resource similar to RapidJSON's pooled allocator.

[nuald]

If you want safer operations, I'd recommend to change get_ to as_ too (i.e. use as_double instead of get_double). I deliberately used unsafe []-operator because we know ahead that the original JSON have the proper format, however in the real life I guess we want to use the safer operations as usually JSON values come across the trust boundaries.

[beached]

At is also faster than operator[] when I was checking. It has a const overload too, so if the object is const it would still work.