improve response context handling of X-Druid-Response-Context header

[himanshug]

currently if responseContext map has non-ascii characters, they will get mapped to ascii possibly to junk characters.
We can probably add a "metadata" attribute in the query response and put the responseContext in there instead of in the metadata.

[vogievetsky]

Did #2336 fix this?

[himanshug]

yes, closing.