Expand uris in $schemas in resolve_all(). (#82)

Author: tetron

schema_salad/ref_resolver.py

@@ -326,7 +326,7 @@ def add_context(self, newcontext, baseuri=""):
             raise validate.ValidationException(
                 "Refreshing context that already has stuff in it")
 
-        self.url_fields = set()
+        self.url_fields = set(("$schemas",))
         self.scoped_ref_fields = {}
         self.vocab_fields = set()
         self.identifiers = set()