title: Robot Framework: Rest call gives "unicode object has no attribute copy" error
body: <p>Below error occurring on my code</p>
  
  <pre><code>${resp}=  Post Request  sw3  https://sw3-   test.test1.com/api/v1/test1/patients
  \  ...  Content-Type:application/json
  \  ...  Authorization:authkey02    Accept=application/json
  </code></pre>
  
  <p>Error as follows</p>
  
  <pre><code>KEYWORD ${resp} = RequestsLibrary . Post Request sw3, https://sw3-test.test1.com/api/v1/test1/patients, Content-Type:application/json, Authorization:authkey02, Accept=application/json
  Documentation:  
  Send a POST request on the session object found using the
  Start / End / Elapsed:  20170315 12:08:21.283 / 20170315 12:08:21.285 / 00:00:00.002
  12:08:21.283    TRACE   Arguments: [ 'sw3' | 'https://sw3-test.test1.com/api/v1/test1/patients' | 'Content-Type:application/json' | 'Authorization:authkey02' | 'Accept=application/json' ] 
  12:08:21.284    FAIL    AttributeError: 'unicode' object has no attribute 'copy'
  </code></pre>
  
  <p>This works ok if i do not use the third line of code - what gives?</p>