fix(http-interceptor): Add boolean to possible return types of Request interceptor
fix(http-interceptor): Add boolean to possible return types of Request interceptor

closes #160

gund/ng2-http-interceptor,gund/ng-http-interceptor,gund/ng2-http-interceptor,gund/ng-http-interceptor,gund/ng2-http-interceptor,gund/ng-http-interceptor