assertTemplateUsed mangles template names in failure message

[davewsmith]

Mistyped a template name in an assertTemplateUsed call, and got

AssertionError: Template ndex.html not used. Templates were used: [ < T e m p l a t e   ' i n d e x . h t m l ' > ]

' '.join(repr(used_templates)) in assertTemplateUsed is the culprit.

PR with fix momentarily.