Update some str methods for recent SDK representation changes

Update Operation.str() to reflect that receivers is now an array
instead of a defaultdict.

Update Worker* objects to not output serialized combine_fn.

These problems surface only when logging at DEBUG level.
----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117727480

google/cloud/dataflow/worker/executor.py

@@ -128,7 +128,7 @@ def str_internal(self, is_recursive=False):
     if not is_recursive and getattr(self, 'receivers', []):
       printable_fields.append('receivers=[%s]' % ', '.join([
           rop.str_internal(is_recursive=True)
-          for oplist in self.receivers.values()
+          for oplist in self.receivers
           for rop in oplist]))
 
     return '<%s %s>' % (printable_name, ', '.join(printable_fields))

google/cloud/dataflow/worker/maptask.py

@@ -60,7 +60,8 @@ def worker_printable_fields(workerproto):
           # want to output value 0 but not None nor []
           if (value or value == 0)
           and name not in
-          ('coder', 'coders', 'elements', 'serialized_fn', 'window_fn',
+          ('coder', 'coders', 'elements',
+           'combine_fn', 'serialized_fn', 'window_fn',
            'append_trailing_newlines', 'strip_trailing_newlines',
            'compression_type', 'context',
            'start_shuffle_position', 'end_shuffle_position',