Added parameter checks to NetClientImpl, CaOptionsImpl, FileSystemImpl, HttpClientImpl, HttpClientRequestImpl, SocketAddressImpl, WebSocketConnectOptionsImpl, added helper class Arguments for argument validation analogous to Objects.requireNonNull(), refactored parameter checks to use consistently Objects.requireNonNull(), added unit tests for several existing parameter checks, fixed small typo in CONTRIBUTING.md.

Signed-off-by: Heribert Hirth <git@frostcode.de>
Merge branch 'parameterchecks' of github.com:heri333/vert.x into heri333-parameterchecks
Merge branch 'heri333-parameterchecks'