Server/Tests: Fix omission from aedb6871 (last commit)

* Needed to correctly support UTF-8 ReadableSTR decoding on Py3

warden_server/test_warden_server.py

@@ -70,7 +70,7 @@ class ReadableSTR(str):
     """Mission: To boldly quack like a buffer, like no str has quacked before"""
     def read(self, content_length=None):
         """Return own content"""
-        return self[0:content_length]
+        return ReadableSTR(self[0:content_length])
 
     if getattr(str, 'decode', None) is None:
         def decode(self, encoding="UTF-8", errors="strict"):    # pylint: disable = locally-disabled, unused-argument