improve(response-rendering): give Content-Disposition render priority

[shockey]

This PR brings Swagger-UI's response rendering logic closer to how browsers handle responses: if Content-Disposition: attachment is set, the user agent should download the response as a file regardless to the content's type.

• Fixes Content-Disposition does not take precedence when rendering responses #4115.
• Resolves Download link does not appear for files with openxmlformats content type #4135.
• cc: Fix: XSLX file operations swagger-js#1227.