Fix registerPush and unregisterPush Content-Type header

Set Content-Type to application/json for registerPush and unregisterPush
endpoints. Previously, contentTypeValue was set to nil, which caused the
Content-Type header to not be set at all. This resulted in PDS rejecting
requests with "Wrong request encoding" error.

The request body was already being JSON-encoded (via toJsonData()), but
without the Content-Type header, the server couldn't properly handle the
request.

Fixes the issue where push notification registration would fail with
400 Bad Request.

Author: P24L

Sources/ATProtoKit/APIReference/AppBskyAPI/AppBskyNotificationRegisterPushMethod.swift

@@ -63,7 +63,7 @@ extension ATProtoKit {
                 forRequest: requestURL,
                 andMethod: .post,
                 acceptValue: "application/json",
-                contentTypeValue: nil,
+                contentTypeValue: "application/json",
                 authorizationValue: "Bearer \(accessToken)"
             )
 

Sources/ATProtoKit/APIReference/AppBskyAPI/AppBskyNotificationUnregisterPushMethod.swift

@@ -61,7 +61,7 @@ extension ATProtoKit {
                 forRequest: requestURL,
                 andMethod: .post,
                 acceptValue: "application/json",
-                contentTypeValue: nil,
+                contentTypeValue: "application/json",
                 authorizationValue: "Bearer \(accessToken)"
             )