compulsory header in javascript code example

Example: accept and content type

Content-Type header is used in POST , PUT , PATCH to specify
the type of body you are sending to the serve and it's required. 

`Accept Header` can be used to ask the server to return
different type of response format 

For example :  

`/api/xname` endpoint , support both xml and json 
as response type so we can add accept header
`application/xml` to get xml result and `application/json` result. 

If server does not support such type ,
it will return `406 not acceptable` status code.