Use JsonSerializer.SerializeToUtf8Bytes when doing a round trip

[Bond-009]

This test uses a very small object (CountryInfo), using a bigger object
would increase the difference in allocated memory.

BenchmarkDotNet=v0.12.1, OS=fedora 32
Intel Core i7-6700HQ CPU 2.60GHz (Skylake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=5.0.100
  [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.51904, CoreFX 5.0.20.51904), X64 RyuJIT
  DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.51904, CoreFX 5.0.20.51904), X64 RyuJIT

|          Method |       Mean |    Error |   StdDev |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|---------------- |-----------:|---------:|---------:|-------:|------:|------:|----------:|
|  RoundTripBytes |   932.0 ns |  5.09 ns |  4.25 ns | 0.1173 |     - |     - |     368 B |
| RoundTripString | 1,114.8 ns | 22.19 ns | 23.74 ns | 0.1469 |     - |     - |     464 B |