addUserWord - limit all inputs to strings only

:Release Notes:
Ensure "addUserWord" accept only string as input

:Detailed Notes:
Ensure "addUserWord" accept only string as input

:Testing Performed:
Was tested on Starfish build 236.

:QA Notes:
correct request:
luna-send -n 1 -f palm://com.palm.smartKey/learn '{"word": "Oulu"}'
wrong request:
luna-send -n 1 -f palm://com.palm.smartKey/learn '{"word": "Oulu@"}'
or
luna-send -n 1 -f palm://com.palm.smartKey/learn '{"word": " "}'

:Issues Addressed:
[GF-10660] com.palm.smartKey:addUserWord - accepting all inputs

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <ikuksyuk@luxoft.com>

Change-Id: I541d72d73c32f77c561eb95f271b6becc77fe957
Reviewed-on: https://g2g.palm.com/2924
Reviewed-by: Build Verification
Reviewed-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Tested-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>