Update most_similar_cosmul to match most_similar

I'm not sure if this fully addresses the
`# TODO: Update to better match & share code with most_similar()`
at line piskvorky#981 or not, so I've left it in.

gensim/models/keyedvectors.py

@@ -989,10 +989,14 @@ def most_similar_cosmul(self, positive=None, negative=None, topn=10):
 
         self.fill_norms()
 
-        if isinstance(positive, str) and not negative:
-            # allow calls like most_similar_cosmul('dog'), as a shorthand for most_similar_cosmul(['dog'])
+        if isinstance(positive, KEY_TYPES + (ndarray,)):
+            # allow passing a single string-key or vector for the positive argument
             positive = [positive]
 
+        if isinstance(negative, KEY_TYPES + (ndarray,)):
+            # allow passing a single string-key or vector for the negative argument
+            negative = [negative]
+
         all_words = {
             self.get_index(word) for word in positive + negative
             if not isinstance(word, ndarray) and word in self.key_to_index