I563: Enable/Disable "Server" header in response. =>

There are some deployment environments that consider leaking the software
  type and/or version to be a security risk. Config.pod mentions that you
  can set "server_tokens" to false to disable the "Server" header, or use
  the environment variable DANCER_SERVER_TOKENS to do the same. These are
  added by this commit.
Change server_tokens to no_server_tokens:

This includes changing DANCER_SERVER_TOKENS to DANCER_NO_SERVER_TOKENS
as well.

Dancer2::Core::Response's BUILD is removed. \o/
reflect change
Merge branch 'feature/optional-server-tokens'