Reported by @sdaftuar (Thanks!).
This fixes a bug that was included in #6121 during the transition from Json Spirit to UniValue.
Verified this fixes getblocktemplate_proposals.py, looks good to me.
Here is the code line before the transition (before #6121) to verify the change: https://github.com/bitcoin/bitcoin/blob/0.11/src/rpcmining.cpp#L403
ACK
utACK
Thanks