Ensure http response wrapper always sets a statusCode

[dmathieu]

If a wrapper doesn't write anything, we don't have any status code available. In which case we can assume it is going to be 200.
This behavior is also what the net/http library assumes if the response writer is a flusher: https://cs.opensource.google/go/go/+/refs/tags/go1.19.1:src/net/http/server.go;l=1702

Closes #2821.

[codecov]

Codecov Report

Merging #2822 (80de95c) into main (e97d789) will increase coverage by 0.0%.
The diff coverage is 89.4%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2822   +/-   ##
=====================================
  Coverage   69.6%   69.6%           
=====================================
  Files        147     147           
  Lines       6785    6795   +10     
=====================================
+ Hits        4725    4735   +10     
  Misses      1944    1944           
  Partials     116     116           

Impacted Files	Coverage Δ
samplers/jaegerremote/sampler_remote.go	87.4% <50.0%> (ø)
instrumentation/net/http/otelhttp/handler.go	81.9% <100.0%> (+0.5%)	⬆️
samplers/jaegerremote/sampler_remote_options.go	100.0% <100.0%> (ø)