Unicode encode problem when saving doc2vec models in plain word2vec format.

[englhardt]

Description

Exporting doc2vec embeddings in plain word2vec format fails when document title contains unicode chars that cannot be ASCII encoded. This only fails in Python 2.7 because Python 3 defaults to unicode strings. The problem is the call str(..) in doc2vec.py

Code to Reproduce

Example:

from gensim.models import doc2vec

# Load doc2vec model with a document title not ASCII encodable
m = Doc2Vec.load('model')
m.save_word2vec_format('model.out')

Expected Results

Export works

Actual Results

---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-3-d12938849014> in <module>()
----> 1 m.save_word2vec_format('model.out')

..../.virtualenvs/gensim27_2.3/local/lib/python2.7/site-packages/gensim-2.3.0-py2.7-linux-x86_64.egg/gensim/models/doc2vec.pyc in save_word2vec_format(self, fname, doctag_vec, word_vec, prefix, fvocab, binary)
    848                 # store as in input order
    849                 for i in range(len(self.docvecs)):
--> 850                     doctag = prefix + str(self.docvecs.index_to_doctag(i))
    851                     row = self.docvecs.doctag_syn0[i]
    852                     if binary:

UnicodeEncodeError: 'ascii' codec can't encode character u'\xf3' in position 8: ordinal not in range(128)

Versions

Linux-4.4.0-59-generic-x86_64-with-Ubuntu-16.04-xenial
('Python', '2.7.12 (default, Nov 19 2016, 06:48:10) \n[GCC 5.4.0 20160609]')
('NumPy', '1.13.1')
('SciPy', '0.19.1')
('gensim', '2.3.0')
('FAST_VERSION', 1)