Work around for property 'Symbol(Symbol.toPrimitive)' in Proxy

[ChuanyuWang]

When I am trying to migrate to higher version nodeJS, e.g. 6.9.1. I found below error coming from Proxy handler, which is caused by Symbol property. So I add another workaround to skip Symbol property during Proxy's get handler

TypeError: '[object Object]' returned for property 'Symbol(Symbol.toPrimitive)' of object '[object Object]' is not a function
    at exports.format (util.js:108:18)
    at Console.log (console.js:43:37)
    at Database.<anonymous> (C:\git\test\app2.js:33:13)
    at emitNone (events.js:86:13)
    at Database.emit (events.js:185:7)
    at C:\git\test\node_modules\mongojs\lib\database.js:35:14
    at connectCallback (C:\git\test\node_modules\mongodb\lib\mongo_client.js:426:5)
    at C:\git\test\node_modules\mongodb\lib\mongo_client.js:364:13
    at _combinedTickCallback (internal/process/next_tick.js:67:7)
    at process._tickCallback (internal/process/next_tick.js:98:9)

[ChuanyuWang]

Close this one and pull another commit to fix the code style