Network Plugin: Add xhtml and xml to 'Textual' check (#2336)

Summary:
Network requests that return a "Content-Type" of 'application/xhtml+xml' are currently being treated as binary data when they should be treated as text.

## Changelog

Network Plugin - treat "Content-Type" of 'application/xhtml+xml' as text

Pull Request resolved: https://github.com/facebook/flipper/pull/2336

Test Plan: Using an API that return "Content-Type" of 'application/xhtml+xml' , verified that the response data in sidebar appears as text rather than '(binary data)'

Reviewed By: nikoant

Differential Revision: D28478695

Pulled By: mweststrate

fbshipit-source-id: ed60c8301dbd0cf65924a80d9e6a5800a59c6bb8

desktop/plugins/public/network/utils.tsx

@@ -39,7 +39,8 @@ export function isTextual(headers?: Array<Header>): boolean {
     contentType.startsWith('application/json') ||
     contentType.startsWith('multipart/') ||
     contentType.startsWith('message/') ||
-    contentType.startsWith('image/svg')
+    contentType.startsWith('image/svg') ||
+    contentType.startsWith('application/xhtml+xml')
   );
 }