Interface encoding with robust memcache fails

I mentioned in the robust memcache thread (#29) that the new code passed all my tests.  That was true at the time, but I've found that I apparently didn't have a very good test for one of my projects and use cases.  When updating goon for github.com/mzimmerman/sdzpinochle I found that this new robust memcache code doesn't properly encode interface types and this commit reflects a simplistic case of how I'm using goon in that project.

@xStrom A fix for the issue wasn't readily apparent to me so I'm hoping you'll know of one :)

Note that the change in serializeStructMetaData() may really just be masking a symptom, but it did remove a panic from this failing code.