Update: Test: Constructor test to cover parameter exceptions
Fix: Visibility of members and methods

    Members and methods() used privately were declared protected. As there
    are no test-stubs there is no use for this.
Fix: Make class final

The class has been declared final to prevent it getting extended.
Update: Switched to SPDX license identifiers
Update: Renamed extension of README.markdown

- .md is more common than .markdown .
Version bump v1.1.13