always use a hard limit for explain()

Author: Mike Dirolf

pymongo/cursor.py

@@ -253,6 +253,10 @@ def explain(self):
  """
  c = self.clone()
  c.__explain = True
+
+ # always use a hard limit for explains
+ if c.__limit:
+ c.__limit = -abs(c.__limit)
  return c.next()
 
  def __index_name_to_list(self, index_name):