Respect explicit format specifiers even when JSON defaults are selected

[nblumhardt]

Fixes #1323

We should check this against the behavior of https://github.com/serilog/serilog-sinks-console/blob/dev/src/Serilog.Sinks.Console/Sinks/SystemConsole/Rendering/ThemedMessageTemplateRenderer.cs before merging.

[nblumhardt]

Okay, so, the mechanism that Console uses here is different; at the top level, it falls back to non-JSON "display" formatting:

https://github.com/serilog/serilog-sinks-console/blob/dev/src/Serilog.Sinks.Console/Sinks/SystemConsole/Formatting/ThemedJsonValueFormatter.cs#L47

Which in turn ends up calling ScalarValue.Render():

https://github.com/serilog/serilog-sinks-console/blob/dev/src/Serilog.Sinks.Console/Sinks/SystemConsole/Formatting/ThemedDisplayValueFormatter.cs#L184

We could try lining these up, but I think rather than try to keep them 100% in sync, it's okay as long as each behaves predictably, at least for now.

A better future design might be to adopt the ConsoleTheme.None formatting into the core Serilog package, somehow, and share the code.

For now, I think this is ready to go.

[tsimbalar]

LGTM ! 👀

[nblumhardt]

Thanks @tsimbalar ! 👍