test_framework: Avoid infinite loop in encoding Decimal

Avoid an infinite loop in encoding, by ensuring EncodeDecimal
returns a string. round(Decimal) used to convert it to
float, but it no longer does in python 3.x. Strings are
supported since #6380, so just use that.

qa/rpc-tests/test_framework/authproxy.py

@@ -61,7 +61,7 @@ class JSONRPCException(Exception):
 
 def EncodeDecimal(o):
     if isinstance(o, decimal.Decimal):
-        return round(o, 8)
+        return str(o)
     raise TypeError(repr(o) + " is not JSON serializable")
 
 class AuthServiceProxy(object):