Fix make_constant() in the AST module, to always return a

constant.  It was failing to do so for timestamps, instead
returning a STIXdatetime object.

stix2/patterns.py

@@ -227,7 +227,7 @@ def make_constant(value):
         return value
 
     try:
-        return parse_into_datetime(value)
+        return TimestampConstant(value)
     except (ValueError, TypeError):
         pass