Fix ResourceProperties.mediaType encoding (#719)

Author: lbeus

CHANGELOG.md

@@ -26,6 +26,8 @@ All notable changes to this project will be documented in this file. Take a look
 
 ### Fixed
 
+* Fixed casting of `ResourceProperties`'s `mediaType` (contributed by [@lbeus](https://github.com/readium/swift-toolkit/pull/719)).
+
 #### Navigator
 
 * The first resource of a fixed-layout EPUB is now displayed on its own by default, matching Apple Books behavior.

Sources/Shared/Toolkit/Data/Resource/ResourceProperties.swift

@@ -55,7 +55,7 @@ public extension ResourceProperties {
         }
         set {
             if let mediaType = newValue {
-                properties[mediaTypeKey] = mediaType
+                properties[mediaTypeKey] = mediaType.string
             } else {
                 properties.removeValue(forKey: mediaTypeKey)
             }