fix MultilineArrayTrailingCommaFixer with heredoc

                    
|Q            |A   |
|---          |--- |
|Bug Fix?     |yes |
|New Feature? |n   |
|BC Breaks?   |n   |
|Deprecations?|n   |
|Tests Pass?  |yes |
|Fixed Tickets|#591|
|License      |MIT |                 

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Good one :+1: