Make nullable getters use Optional<T> or Option[T]

[wsargent]

In many places, the Java WSRequest and WSResponse still use methods which return null:

public interface StandaloneWSRequest {
    String getUsername();

    String getPassword();

    WSAuthScheme getScheme();

    WSSignatureCalculator getCalculator();

    Duration getRequestTimeoutDuration();

    boolean getFollowRedirects();

    String getContentType();
}

And they should return Optional<String> etc where appropriate.

[wsargent]

Addressed by #129

[dmitraver]

hi, is this one up for picking or is it already fixed by #129 ?

[marcospereira]

Hey, @dmitraver,

it was already addressed by #129.

Best.