httpserver: do not override response code in request post-processing phase

Fixes issue with response code set from a lambda handler getting
overriden with '200 OK'.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
httpserver: cleanup: remove unused parameter from reply400/reply500

This parameter has no effect and only gives a false ilusion that error
code can be controlled with it.

Fixes #58.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
httpserver: cleanup: make handle() return void

The return value of handler_base::handle() is not used for anything.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>