Fix trace_block for stateDiffTracer

[ziogaschr]

Fix trace_block when used with the stateDiffTracer, as the trace_block was expecting an array while stateDiff returns a JS object.

This has been tested on classic.

Before fix:

> trace.block('0x4BAF8', {"tracer":"stateDiffTracer"})
Error: method handler crashed
        at web3.js:6347:37(47)
        at web3.js:5081:62(37)
        at <eval>:1:12(7)

After fix:

> trace.block('0x4BAF8', {"tracer":"stateDiffTracer"})
[{}, {
    action: {
      author: "0x7e6930b63d03e88f5928c5cd6ee3fcee4d714f7f",
      rewardType: "block",
      value: "0x4563918244f40000"
    },
    blockHash: "0x8cd39d130fd3eb24296c571a5d4bf29f47b236538e297e1b009fae8ad0b9ffbe",
    blockNumber: 310008,
    result: null,
    subtraces: 0,
    traceAddress: [],
    transactionHash: null,
    transactionPosition: null,
    type: "reward"
}]