Properly encode content-disposition header

[weierophinney]

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Always use ASCII for the content-disposition header. When using UTF-8 encoding, this results in a header being set as Content-Disposition: =?utf-8?Q?inline?= which is not interpreted properly by Thunderbird nor Microsoft Exchange / Outlook. This pull request forces encoding for this header to always be ASCII, so that the header reads Content-Disposition: inline and is correctly interpreted by email clients.

Fixes: magento/magento2#25076

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Originally posted by @fredden at zendframework/zend-mail#251