fixed bug for setValue in refreshAccessToken

fixed bug for setValue in refreshAccessToken

Since OCRepresentation doesn't perceive a const string as std::string value,
need to clarify a type of parameter.
Otherwise, it is passed as a true boolean value whatever it is.

Change-Id: I3db677f6bdf59bf23036dc678781b35af95025e6
Signed-off-by: Jaewook Jung <jw0213.jung@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/10539
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jee Hyeok Kim <jihyeok13.kim@samsung.com>