Return 405 when attempting DELETE on /tasks

We currently return 404 NotFound when attempting to delete a task
by hitting the resource /v2/tasks/{task_id}. However, as this
resource exists and we do not allow DELETE on tasks, the correct
response is 405 MethodNotAllowed.

Implementing as per spec on
http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

Fixes bug 1287951

Change-Id: Ia4a969e595b520dbc5f4e9a463d967681465ae6f
Merge "Return 405 when attempting DELETE on /tasks"