Is the max current draw of wall outlets reported in RMS or peak?
Like Brian said, it's measured in RMS.

For the extension: RMS again.  For fast-blow fuses, the current rating is the maximum current guaranteed at which the fuse will not blow.  But the fuse won't immediately blow at 15.1A -- the time vs. current graphs are provided for most fuses.  Here's an example:



As we can see, a 15A fuse is guaranteed not to blow at 15A.  As the current increases, the time it takes for the fuse to blow decreases.  A 15A fuse may take a minute to blow at 20A, but 0.3 seconds to blow at 30A.

Even though your 15Arms current is actually over 21A peak, the time that it goes above the 15A rating (~4ms per half cycle) is not sufficient to blow the fuse.  Due to the fuse's thermal inertia you may want to derate the curve a bit, but at 15Arms you should be fine.