title: Spring security check is header present with some fixed value
body: <p>Let's say I have spring boot rest service which I would like to secure with this simple logic, 
  if there is header "token" with a value "123456" then request is fine, and I should process it,
  if not then I should sent back unauthorized 401 http error. </p>
  
  <p>How I can implement this logic in spring boot 1.5.</p>