fixed get_keras_embedding, now accepts word mapping (#2676)

* fixed get_keras_embedding, now accepts word mapping

* skip tests if keras not installed

* removed unnessecary comment from test_keyed_vectors

* fixed indentation

* fixed flake import error

* moved skip test decorator to class

* Update gensim/models/keyedvectors.py

Co-Authored-By: Michael Penkov <m@penkov.dev>

* Update gensim/models/keyedvectors.py

Co-Authored-By: Michael Penkov <m@penkov.dev>

* Update gensim/models/keyedvectors.py

Co-Authored-By: Michael Penkov <m@penkov.dev>

* renamed keras_installed flag to upper case, removed unneeded comment

Co-authored-by: Zhicharevich <Alex_Zhicharevich@intuit.com>
Co-authored-by: Michael Penkov <m@penkov.dev>