Fix the attributes of Attachment (#5)

* Fix Attachment.text_url not being nullable

* Add missing "unknown" attribute to MediaType, which is not in the documentation by TootSuite

https://github.com/tootsuite/mastodon/blob/b97ebaf6202debb8c24d6f726886a594f02a6d3b/app/models/media_attachment.rb#L24

src/entities/attachment.rs

@@ -6,7 +6,7 @@ pub struct Attachment {
     pub url: String,
     pub remote_url: String,
     pub preview_url: String,
-    pub text_url: String,
+    pub text_url: Option<String>,
 }
 
 #[derive(Debug, Deserialize, Clone, Copy)]
@@ -17,4 +17,6 @@ pub enum MediaType {
     Video,
     #[serde(rename = "gifv")]
     Gifv,
+    #[serde(rename = "unknown")]
+    Unknown,
 }