Add `application/rdf+xml` to harmless mimetypes

src/harmless.py

 from . import abstract
 
 class HarmlessParser(abstract.AbstractParser):
-    mimetypes = {'application/xml', 'text/plain'}
+    mimetypes = {'application/xml', 'text/plain', 'application/rdf+xml'}
 
     def __init__(self, filename: str):
         self.filename = filename