* Fix#1824: HEAD request on static files causes app to stop
The problem was that for HEAD requests specifically, we implement
an explicit Content-Length header (normally we let httpuv figure
out the Content-Length based on the content, but for HEAD we don't
return any content but still want to include the Content-Length).
The Content-Length header was only implemented correctly for string
values, not for raw vectors or file-by-path. This change implements
the value correctly for all currently valid httpuv content.
* Update NEWS
* Code review feedback
- More selective removal of frames at end of call stack
- Add withLogErrors, printError, stripStackTrace convenience funcs
- Properly capture/log errors for various levels of unhandled errors
- Unhide stacks for flush/flushed/sessionended callbacks
