text: Title: Concatenating variables containing bytes should yield bytes
  Body: When joining bytes with ${x}${y} string is returned instead of bytes. 
  
  consider following test case:
  ```robot
  *** Test Cases ***
  Join bytes
      ${destination} =    Convert To Bytes    FF FF FF FF FF FF    hex
      ${source} =         Convert To Bytes    00 80 E1 00 00 00    hex
      ${data} =           Convert To Bytes    00 01 08 00 06 04 00 01     hex
      
      ${frame} =   Set Variable    ${destination}${source}${data}
  ```
  currently results in `${frame} -> '\\xff\\xff\\xff\\xff\\xff\\xff\x00\\x80\\xe1\x00\x00\x00\x00\x01\x08\x00\x06\x04\x00\x01'`
  expected result is bytes