fixed index name error found at redis new vector test (#2311)
This PR fixes a logic error in the Redis VectorStore class
Creating a redis vector store `from_texts` creates 1:1 mapping between
the object and its respected index, created in the function. The index
will index only documents adhering to the `doc:{index_name}` prefix.
Calling `add_texts` should use the same prefix, unless stated otherwise
in `keys` dictionary, and not create a new random uuid.
