title: Using httr package, setting a header where the header name is a variable
body: <p>I want to set a header in an request using the R httr package and set a header, when I have the name of the header in a variable.</p>
  
  <p>I would like to do something like this:</p>
  
  <pre><code>tokenName = 'X-Auth-Token'
  
  get_credentials_test &lt;- function (token) {
  
    url &lt;- paste(baseUrl,"/api/usercredentials", sep = '')
    r &lt;- GET(url, add_headers(tokenName = token))
    r
  }
  </code></pre>
  
  <p>however, the above code seems to set a header with the name tokenName.</p>
  
  <p>It does work if I do the following:</p>
  
  <pre><code>get_credentials_test &lt;- function (token) {
  
     url &lt;- paste(baseUrl,"/api/usercredentials", sep = '')
     r &lt;- GET(url, add_headers('X-Auth-Token' = token))
     r
  }
  </code></pre>
  
  <p>but I want to have some flexibility if the name of the header changes and the requirement to add the header is sprinkled liberally around the code.  I am not sure if it is possible to add a header contained with a variable but that is what I would like to do.</p>