message: Make CosmosMsg::Contract.send optional
diff: @@ -110,7 +110,7 @@ pub enum CosmosMsg {
  Contract {
  contract_addr: HumanAddr,
  msg: String,
  - send: Vec<Coin>,
  + send: Option<Vec<Coin>>,
  },
  // this should never be created here, just passed in from the user and later dispatched
  Opaque {
  