I agree. What you are seeing appears like a bug. The wizard should automatically encode URL unsafe characters. Surprisingly, I retested your original URL (without %20) in 7.1.7 and it seems to work fine. Here is the URL to the reference system. http://vref71ml7:7350/v71ml7/PRServlet
I see what you must have done to replicate the problem. Have you entered the query string value in the test dialog directly instead of starting off the wizard with the complete URL? When I did that, I see the problem you are seeing. That's a bug we fixed in 7.1.8.
See my update below that post. I didn't change the value in the query string field. I entered your original URL https://www.googleapis.com/books/v1/volumes?q=Software Design + intitle:Architecture&maxResults=1&printType=books in the first step of the wizard (Enter URL). That ‘+' was automatically added in the subsequent steps and so it worked in my case. If I replace + with space, it would fail as you observed. That's still a bug though that we fixed in 7.1.8.