hi i am sending only a single Http Request in Batch but still facing error System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out

My test code is 

@isTest(SeeAllData=true)
static void test1(){
MirrorTestUtil.setupSettings();
User u = [Select Authorize__c From User Where Id = :UserInfo.getUserId()];
u.Authorize__c = true;
update u;
System.debug('u is '+u);

List<FeedItem> feedList= new List<FeedItem>();
for (Integer i = 0; i<5;i++)
feedList.add( new FeedItem(Body='@'+UserInfo.getName()+'Hello World'+i,ParentId = UserInfo.getUserId()));
Test.setMock(HttpCalloutMock.class, new MirrorMockTimelinePostImpl());
Test.startTest();
Test.setMock(HttpCalloutMock.class, new MirrorMockTimelinePostImpl());

insert feedList;
Test.stopTest();
}

and My trigger code is 

trigger PostFeedsToTimeLine on FeedItem (after insert) {

BatchPublishTimeLine publishBatch = new BatchPublishTimeLine(Trigger.new,contentMap);
Database.executeBatch(publishBatch,5);
}

and My Batch class code is 

public class BatchPublishTimeLine implements Database.Batchable<sObject>{
sObjectIterable iterable;
Map<Id,User> userMap;
Map<String,String> contentmap;

public BatchPublishTimeLine(List<sObject> objectList,Map<String,String> contentmap){
iterable = new sObjectIterable(objectList);
this.userMap = new Map<Id,User>([Select Id,Name From User WHERE Authorize__c = true]);
this.contentmap = contentmap;
}

public Iterable<sObject> start(Database.BatchableContext BC){
return iterable;
}

public void execute(Database.BatchableContext BC, List<sObject> scope){
GMirrorUtil.createTimeLine(scope, contentMap);
}

public void finish(Database.BatchableContext BC){
System.debug('Job Has been Finished');
}
}

and My createTimeLine function code is 

public static void createTimeLine(List<sObject> objList,Map<String,String> contentMap){

String timelineRes = doApiCall('xyzzz','POST','https://www.googleapis.com/mirror/v1/timeline','xxxxxxxxx');

}

doApiCall code is 

public static String doAPICall(String postBody, String method, String endPoint, String accessToken){

HttpRequest req = new HttpRequest();
Http http = new Http();
HttpResponse res;

req.setEndpoint(endPoint);
req.setMethod(method);
req.setHeader('Content-Type','application/json');

if(method == 'POST' || method == 'PUT')
req.setBody(postBody);
req.setHeader('Authorization','Bearer ' + accessToken);
res = http.send(req);
String result = res.getBody();
System.debug('status code is '+res.getStatus());
System.debug('result is'+res.getBody());
return result;
}

and the Mock class code response is 

@isTest
global class MirrorMockTimelinePostImpl implements HTTPCalloutMock {
global HTTPResponse respond(HTTPRequest req) {
// Optionally, only send a mock response for a specific endpoint
// and method.
System.assertEquals('https://www.googleapis.com/mirror/v1/timeline', req.getEndpoint());
System.assertEquals('POST', req.getMethod());
System.assert(req.getHeader('Authorization').startsWith('Bearer'));

// Create a fake response
HttpResponse res = new HttpResponse();
res.setHeader('Content-Type', 'application/json');
res.setBody('{"kind":"mirror#timelineItem", "id":"mockid", "created":"2013-07-31T12:07:34.882Z", "updated":"2013-07-31T12:07:34.882Z", "etag":"\\"ZECOuWdXUAqVdpmYErDm2-91GmY/NVMWuR8LJyCKttsmne9R4K8n7YI\\"", "text": "New Lead: OauthCheck EarlyAm, Google, Inc., (234) 567-8900"}');
res.setStatusCode(200);
return res;
}
}

and i am unable to figure it out how using only one callout its throwing an error ?? Please help