Updated javadoc information.
Spockified one of the tests before making changes.
Added integ test coverage for the javadoc task for 'header' property of the javadoc options.
GRADLE-3099 Javadoc options now support multiline values.

This is especially useful for certain html javadoc options like the 'header'. Header typically contains html: scripts or some javascript that enhances the javadoc usability. We could support multilines only for specific options, for example those that contain html content. However, supporting multilines for all options is a simple improvement and we can grow it / change it if needed.
Removed a method that was not used anywhere.