explicitly stringify values at "input"

user can pass in a dict(); os.environ always contains string values, though,
so make sure that the dict() looks like what would be returned from os.environ

tldp/cascadingconfig.py

@@ -174,7 +174,7 @@ def dict_from_envdict(env=os.environ, base=None, envsep=ENVSEP, clisep=CLISEP):
     for k, v in env.items():
         if k.startswith(tag):
             k = k.lower().replace(envsep, clisep)
-            d[k] = v
+        d[k] = str(v)
     return d