A fix and optimization for the HTTP interface to riak_kv

The fix is just dropping the ability to have the user set "content-encoding" and subsequently stop storing it on the riak_object.  It seems to have possibly be previously conflated as another sort of "content-type" metadata, but the HTTP RFC disagrees with us.

The optimization is to avoid the current code path on PUT that does a full client replica GET request at the coordinator before performing the actual PUT operation.  Testing done by @deanproctor showed that on a 100% PUT load we could cut CPU, network, and disk utilization almost in half.