Depends on instance attribute `hasOwnProperty`

[jdthorpe]

These calls all throw the error TypeError: object.hasOwnProperty is not a function:

sizeof = require('object-sizeof')
sizeof({hasOwnProperty:undefined})
sizeof({hasOwnProperty:"Hello World"})
sizeof({hasOwnProperty:1234})

For greater robustness, I recommend changing this:

if(!object.hasOwnProperty(key)){
    continue;
}

to the more general:

if(!Object.hasOwnProperty.call(object,key)){
    continue;
}

[miktam]

thank you, @jdthorpe
fixed #14