JSON documents can't be compressed

In SPY-58 compressing for JSON was disabled. We picked spymemcached because it did compression and couldn't find any documentation saying JSON wouldn't be compressed. Whether to compress JSON or not should be a configuration value that can be passed in much like compressionThreshold.

Currently our only solution appears to prepend one character that isn't { or [ and then remove it after doing a get. But I would say that not compressing any string just because it starts with a { or [ isn't really desirable because it means that if I expect/want compression of my string I now have to check every string I put into the client and prepend and remove a leading character just to get compression.