[webapi] Notification iconPath value verification fixed

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [IVI] crosswalk-10.38.222, t-e-c 0.107
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #1145 from xinliux/notification-iconPath

[webapi] Notification iconPath value verification fixed