Server/Tests: Make ReadableSTR.read() understand the length parameter

warden_server/test_warden_server.py

@@ -68,9 +68,9 @@ def tearDownModule():  # pylint: disable = locally-disabled, invalid-name
 
 class ReadableSTR(str):
     """Mission: To boldly quack like a buffer, like no str has quacked before"""
-    def read(self, content_length=0):   # pylint: disable = locally-disabled, unused-argument
+    def read(self, content_length=None):
         """Return own content"""
-        return self
+        return 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