Fix ResponseParser for InfluxDB to return only string values

public/app/plugins/datasource/influxdb/response_parser.ts

@@ -45,7 +45,7 @@ export default class ResponseParser {
     });
 
     return _.map(res, value => {
-      return { text: value };
+      return { text: value.toString() };
     });
   }
 }

public/app/plugins/datasource/influxdb/specs/response_parser.jest.ts

@@ -105,9 +105,9 @@ describe('influxdb response parser', () => {
 
     it('should return second column', () => {
       expect(_.size(result)).toBe(3);
-      expect(result[0].text).toBe(0);
-      expect(result[1].text).toBe(15);
-      expect(result[2].text).toBe(20.2);
+      expect(result[0].text).toBe('0');
+      expect(result[1].text).toBe('15');
+      expect(result[2].text).toBe('20.2');
     });
   });