Implement set/delete of assignees

[PMooreWand]

Adding methods and tests for setting
and deletion of assignee from a change.

Added extra expectDelete method to handle
passed in result object.

[PMooreWand]

Apologies there may be more of these over coming weeks, we're in middle of rewriting our codebase for gerrit 3.7 and moving to api / ssh calls to reduce the about of selenium rewrites to handle Lit.

[uwolfer]

Apologies there may be more of these over coming weeks, we're in middle of rewriting our codebase for gerrit 3.7 and moving to api / ssh calls to reduce the about of selenium rewrites to handle Lit.

Thank you very much - appreciated!