Strip supplied `value`, `type`, and `subtype` in `MediaType`

[sbrannen]

Overview

In order to support scenarios where the user inadvertently supplied a value, type, or subtype with leading or trailing whitespace, we should consistently strip a supplied value, type, or subtype in MediaType like we do elsewhere in the code base.

Related Issues

• Reject blank value, type, or subtype in MediaType #4895