Fixes for RMI and JSONBaseMixin

When I was testing the new `djng_current_rmi` template tag, I was invariably receiving an `ImproperlyConfigured` exception (message: *[...] doesn't look like a module path*). Additionally, the implementation was accessing `context['request']`, which depends on `django.core.context_processors.request` to be added to `TEMPLATE_CONTEXT_PROCESSORS`. That is very common but not the default.
I noticed that `djng_all_rmi` is using a different approach, but since the view instance is already part of the context without further configuration, it can also be used for finding the RMI methods (commit 3a8f922).

Commit 222ca3e fixes a problem that was introduced with my PR #37. It occurs when the mixin is used with a view generating HTML content, and not only JSON.