allow maxlength validation for inputs which can be blank

[kv109]

Before:

validates :company_url, allow_blank: true, length: { maximum: 255 }

was translated to:

<input type="url" name="company[company_url]" id="company_company_url" />

Now it's translated to:

<input maxlength="255" size="255" type="url" name="company[company_url]" id="company_company_url" />

[amatsuda]

@kv109 Thanks!