Validate the Content-Type on PUT requests

Return a 415
(https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/415) when the
Content-Type does not look like a valid MIME type (in the type/subtype
format)

Refs #137

lib/remote_storage/rest_provider.rb

@@ -506,5 +506,10 @@ module RemoteStorage
       items
     end
 
+    def validate_content_type(content_type)
+      # Do not try to perform the PUT request when the Content-Type does not
+      # look like a MIME type
+      server.halt 415 unless content_type.match(/^.+\/.+/i)
+    end
   end
 end