IMutableFileNode is a subtype of IFileNode

I'm not 100% sure that this is correct, but it looks reasonable, it passes unit
tests (although note that unit tests are currently not covering the new mutable
files very well), and it makes the "view JSON" link on a directory work instead
of raising an assertion error.

src/allmydata/interfaces.py

@@ -473,7 +473,7 @@ class IFileNode(Interface):
     def check():
         """Perform a file check. See IChecker.check for details."""
 
-class IMutableFileNode(Interface):
+class IMutableFileNode(IFileNode):
     def download_to_data():
         """Download the file's contents. Return a Deferred that fires with
         those contents. If there are multiple retrievable versions in the