Fix UnicodeEncodeError in csv export: Bug 1044094

> IMO just creating some signed agreements in the DB would be better than all this trouble for mocking. 

I realize that I'm very much in the minority when it comes to my opinions on the merits of fully isolated unit tests, and that most of the benefits will not be realized in this project due to most of the tests are being written by others who do not share my views, but I'm going to continue trying, if you don't mind. :) When you get a chance, please read the [Gary Bernhardt's response](http://www.obeythetestinggoat.com/fast-tests-useless-hot-lava-be-damned.html#comment-1042970785) to [an article that takes a similar position](http://www.obeythetestinggoat.com/fast-tests-useless-hot-lava-be-damned.html) and the exchange that follows.

> Barring that, simplifying would help; perhaps use a SignedAgreement instance instead of creating the mock signed agreement, and maybe instead of using MockQS just make a normal mock as the return value for all that simulates a queryset?

Yes, I would have preferred that solution as well, and indeed tried each of those solutions before I came to the one in this PR, but due to some unfortunate design decisions none of that will work with the code as it is, and this PR is about fixing unicode errors, not design decisions, but I'd be happy to discuss that further with you later.