Description

Fixes stream usage for cudf::string_scalar variables used in write_json.cu. The passed-in stream should be used instead of the default stream supplied by the string_scalar constructor. Also removed some unused variables. A couple of strings are hardcoded and so these were moved to device code as constants to avoid the host-to-copy that occurs within the string_scalar constructor.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.