fix vue2.0 checkbox cannot display bug

[yugasun]

when I use vuetable-2 for vue2.0, I have config thefields attribute, but checkbox cannot display. After read the source code I found that it still use props mutable, but vue2.0 has deprecated (2.0 Changes). So I add a middle parameter tableFields to act as the mutable data.

[ratiw]

Thanks, @yugasun for the PR. However, this bug has already been fixed and has nothing to do with the fields attribute. Vue does not throw any warning or error with the way I reuse fields prop right now. If you clone and run the project, you should see no error from Vue regarding this.

But I'm quite curious why adding tableFields variable and use it instead of the old fields prop does solve your problem with the checkbox as I cannot see any relationship on this. Furthermore, the checkbox field in my example is still working correctly.

[yugasun]

@ratiw , In my project I init the vuetable attribute fields, but the fields need change depending on the function loadSuccess data, so I have to change the props fields, but in > vue2.0 mutable props has been deprecated. So I add this PR for my project.

[ratiw]

@yugasun This is to let you know that mutating fields prop inside Vuetable is now actually a bug as reported and discussed in #51 when directly assigning an array to fields prop. This makes the code in normalizeFields method behaves incorrectly. This does not occur when assigning an array object to fields prop.