Workspaces Aren't Fully Destroyed

Calling Graph.destroyWorkspace().named(String) is supposed to destroy the workspace, but right now it only deletes the content from the workspace and doesn't remove the workspace from the list of workspaces named by the repository source. Among other things, this causes the following code to not work:

Graph graph = ...;

graph.createWorkspace().named(workspaceName);

graph.useWorkspace(defaultWorkspaceName);
graph.destroyWorkspace().named(workspaceName);

graph.createWorkspace().named(workspaceName);

The second createWorkspace call fails because the repository object in the source never deleted its reference to the workspace named workspaceName. This behavior is consistent across all repository sources.