[SPARK-17548][MLLIB] Word2VecModel.findSynonyms no longer spuriously rejects...

[SPARK-17548][MLLIB] Word2VecModel.findSynonyms no longer spuriously rejects the best match when invoked with a vector

## What changes were proposed in this pull request?

This pull request changes the behavior of `Word2VecModel.findSynonyms` so that it will not spuriously reject the best match when invoked with a vector that does not correspond to a word in the model's vocabulary.  Instead of blindly discarding the best match, the changed implementation discards a match that corresponds to the query word (in cases where `findSynonyms` is invoked with a word) or that has an identical angle to the query vector.

## How was this patch tested?

I added a test to `Word2VecSuite` to ensure that the word with the most similar vector from a supplied vector would not be spuriously rejected.

Author: William Benton <willb@redhat.com>

Closes #15105 from willb/fix/findSynonyms.