Fixes web3.sha3 leading '0x' compatibility issue between <= 0.15 and >= 0.16 versions

fix-web3-sha3.js

var Web3 = require('web3');
var web3 = new Web3();

function fixSha3(web3) {
  var original = web3.sha3;
  web3.sha3 = function() {
    var result = original.apply(this, arguments);
    if (result[1] !== 'x') {
      return '0x' + result;
    }
    return result;
  }
  return web3;
}

console.log(web3.sha3('asdf'));
fixSha3(web3);
console.log(web3.sha3('asdf'));