add minlength as valid attribute for textarea

[weierophinney]

According to MDN web docs and WHATWG HTML Living Standard, the minlength attribute also apply to textarea form elements.

other form elements that applied minlength already included by zendframework/zend-form#206

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Originally posted by @habibillah at zendframework/zend-form#231

[weierophinney]

We should add this change to Changelog file.

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Originally posted by @dennybrandes at zendframework/zend-form#231 (comment)

[samsonasik]

I created PR #69 for it.