Http call response header returning null in Ionic 2

Ionic Framework ionic-v3
1

IOnic serve case:
when i run ionic serve http call return null in response header. i need headers to send in every next call for secure authentication. but Api returning null in response header when i call login http call.

Ionic run android case:
But another case is when i run ionic run android it give me response header in login call but right after the login call when i call next http call it returns null in response header of next call.

Here is my Login code:

 Login(email, password) {

        let body = JSON.stringify({email: email, password: password});
        //HTTP POST CALL FOR LOGIN 
        return this.http.post(this.LOGIN_URL, body, {headers: this.contentHeader}).subscribe(response => {

            if (response) {
         
                console.log('in login if');
                console.log(response.headers.get('uid'));
                console.log(response.headers.get('client'));
                console.log(response.headers.get('access-token'));

          this.uid = response.headers.get('uid');
          this.client =  response.headers.get('client');
          this.access_token =  response.headers.get('access-token');

//saving header for next call
  Save_Header(this.access_token, this.client, this.uid );

       
            } else{
              // login failed
              console.log("coming in........");
              this.error = 'Something went wrong';
              //  this.loading = false;
            }
    }

Here is function which saving headers for next call:

Save_Header(access_token,client, uid){
        this.contentHeader.append('access-token', access_token);
        this.contentHeader.append('client', client);
        this.contentHeader.append('uid', uid);
       //console.log(this.contentHeader);
    }

function for getting header for next call:

   Get_Header(){
    return this.contentHeader;
}

Next http call:

Get_All_Delivered_orders(){
        console.log("in userservice all delivered orders")
        console.log(this.Get_Header());
       return this.http.get(URL,{headers: this.Get_Header()});// getting saved headers
    }