cubicweb #3677949 HEAD always returns 200 [validation pending]

our HEAD request handling is at odds with the spec. we always return 200, and miss a bunch of response headers.

prioritynormal
typebug
done in3.20.0
load0.250
load left0.000
closed by#e0b3fcf1c205 [web] Fix HEAD request handling (closes #3677949)
patch[web] Fix HEAD request handling (closes #3677949) [applied]