Explicitly cast value as string.

[dcode]

Since the value could be any valid Java value type, cast it to a string. Without this, Elasticsearch cannot generate a valid mapping, since it could be an int in one record and a string in the next.

NOTE: not sure if this is needed anymore w/ the other changes. If not, just close the PR and I'll delete my fork.

[compsecmonkey]

Good call. Thanks for the PR