So the AssetServlet had a memory leak? Serving stuff from nginx is undoubtedly more efficient, but it would have been nice to know the real cause instead of just the workaround. If you work around a problem it has the nasty tendency to come back to bite you at another time.
Given the sophistication of the HotSpit JIT compiler, I'm surprised that zlib hasn't yet been replaced with a pure-Java implementation in the JRE. Of course, I'm presuming that a pure-Java implementation would be much less likely to leak.