Set the document type in the document serializer to read only.

This change improves PUT requests and changing the document type
via the API is not supported anyways.

mayan/apps/documents/serializers.py

@@ -161,6 +161,7 @@ class DocumentSerializer(serializers.HyperlinkedModelSerializer):
             'latest_version', 'url', 'uuid', 'versions',
         )
         model = Document
+        read_only_fields = ('document_type',)
 
 
 class NewDocumentSerializer(serializers.ModelSerializer):