The XMLHttpRequest method setRequestHeader() sets the value of an HTTP request 
header. When using setRequestHeader(), you must call it after calling open(), 
but before calling send(). If this method is called several times with the 
same header, the values are merged into one single request header.

Each time you call setRequestHeader() after the first time you call it, 
the specified text is appended to the end of the existing header's content.
XMLHttpRequest allows both to send custom headers and read headers 
from the response.

setRequestHeader(name, value)
xhr.setRequestHeader('Content-Type', 'application/json');