Billionaire David Rubenstein is going to pop the question. The HQ2 question. The president of The Economic Club of Washington, D.C., gave me a preview about his upcoming Sept. 13 dinner conversation with Amazon.com founder and CEO Jeff Bezos — when many minds are focused squarely on the will-he-or-won’t-he game simply called HQ2 and the promise of tens of thousands of tech jobs and millions of square feet. “That will be the first question probably,” Rubenstein told me in an exclusive interview…