`hasStyle` prints wrong error when value is not a string

[austindebruyn]

What I did:

Called hasStyle with an integer for a pixel-specified rule

    wrapper = mount(alert, { propsData: { top: 10, message: 'hey' }})
    expect(wrapper.hasStyle('top', 10)).to.be.true

Expected output:

    [avoriaz]: wrapper.hasStyle() must be passed value as string
    error@http://localhost:9877webpack:///node_modules/avoriaz/dist/avoriaz.js:136:0 

Actual output:

    [avoriaz]: wrapper.hasClass() must be passed value as string
    error@http://localhost:9877webpack:///node_modules/avoriaz/dist/avoriaz.js:136:0 

My fix: #156