Fix to override existing tags #2536

[valllabh]

• This fix honors user added tags in order they appear and overrides previous tags if any
• tags are now sorted in alphabetical order to have more consistency
• added extra tests post model generation

@StevenACoffman you might want to check this

I have:

• [Yes] Added tests covering the bug / feature (see testing)
• [Not Required] Updated any relevant documentation (see docs)

[coveralls]

Coverage: 75.576% (+0.003%) from 75.574% when pulling 4131f89 on valllabh:master into 356f4f9 on 99designs:master.