default headers now are getting from class property in HttpError
HttpError's headers property reworked
HttpError: casting .title() for default headers too
yet another HttpError's headers property reworking
list replaced with set in 'update' method
adding Content-Length to headers only if it wasn't set earlier
final 'headers' of HttpError
'in' support for response headers
Handler: don't set Content-Length if it was already set
Handler: default Content-Type