Avoid using context.request directly

[Jake Howard]

The RequestContext subclass sets context.request to the current request object. This attribute isn't available when Context is used, or when something context-looking is used instead. For example, the new querystring tag accesses context.request, making it incompatible in some cases.

I'd suggest that uses of context.request directly be replaced with context["request"], and that RequestContext automatically add request to its own context if it's not there already. It might also be worth renaming the attribute to _request to hint it shouldn't be used externally.

This likely only occurs when template context is being incorrectly handled, but it's likely worth improving regardless.

[jaffar Khan]

Like to work on it if accepted.