Chapter 7 update

As a couple different readers noted, there was a mistake in the flow of Chapter 7 of the Ruby on Rails Tutorial book: the error messages shown in Figure 7.5 didn't actually appear in the tutorial as written, since there was no call to @user.save in the create action to generate them. In the process of fixing this problem, I realized there were other structural issues that needed to be addressed, including properly test-driving the failing create action and introducing message expectations a little earlier. The result is that Section 7.2: Signup failure has been substantially rewritten, and Section 7.3: Signup success has been revised as well. If you've already worked through Chapter 7, you might want to revisit the updated sections so that you'll be ready for Chapter 8, which I hope to release some time next week.