text: Title: Merging corpora requires converting itertools chain object to list object
  Body: When merging corpora, it is essential to convert the itertools.chain object to a list. Otherwise the serialization will not save the older corpus.
  
  # now we can merge corpora from the two incompatible dictionaries into one
          merged_corpus = itertools.chain(some_corpus_from_dict1, dict2_to_dict1[some_corpus_from_dict2])
  
  should be
  
  merged_corpus = list(itertools.chain(some_corpus_from_dict1, dict2_to_dict1[some_corpus_from_dict2]))
  
  Then the merged_corpus can be serialized using the standard
  
  MmCorpus.serialize(merged_corpus_output_fname, merged_corpus)