simplify BlockReward::reward implementation

[debris]

reduce number of expects

[ordian]

Tests need to be fixed:

error[E0308]: mismatched types
   --> ethcore/src/engines/block_reward.rs:207:40
    |
207 |         assert!(block_reward_contract.reward(&vec![], &mut call).unwrap().is_empty());
    |                                              ^^^^^^^
    |                                              |
    |                                              expected struct `std::vec::Vec`, found reference
    |                                              help: consider removing the borrow: `vec![]`
    |
    = note: expected type `std::vec::Vec<(ethereum_types::H160, engines::block_reward::RewardKind)>`
               found type `&std::vec::Vec<_>`

error[E0308]: mismatched types
   --> ethcore/src/engines/block_reward.rs:216:46
    |
216 |         let rewards = block_reward_contract.reward(&beneficiaries, &mut call).unwrap();
    |                                                    ^^^^^^^^^^^^^^
    |                                                    |
    |                                                    expected struct `std::vec::Vec`, found reference
    |                                                    help: consider removing the borrow: `beneficiaries`
    |
    = note: expected type `std::vec::Vec<(ethereum_types::H160, engines::block_reward::RewardKind)>`
               found type `&std::vec::Vec<(ethereum_types::H160, engines::block_reward::RewardKind)>`

error: aborting due to 2 previous errors

[niklasad1]

LGTM, but the tests need to be fixed as Andronik already said