Decode server responses from raw bytes to strings

Uses utf-8, or whatever the server response indicates.

tests/test_nilmdb.py

@@ -136,8 +136,9 @@ class TestBlockingServer(object):
         thread.join()
 
 def geturl(path):
-    req = urlopen("http://127.0.0.1:32180" + path, timeout = 10)
-    return req.read()
+    resp = urlopen("http://127.0.0.1:32180" + path, timeout = 10)
+    body = resp.read()
+    return body.decode(resp.headers.get_content_charset() or 'utf-8')
 
 def getjson(path):
     return json.loads(geturl(path))