Base64 encoding/decoding (via stout) behaves differently on Windows

C:\Users\Administrator\workspace\mesos\Mesos_CI-build\FLAG\Plain\label\mesos-ec2-windows\mesos\3rdparty\stout\tests\base64_tests.cpp:60
Value of: base64::encode_url_safe("~~~\u00ff\u00ff\u00ff\u00ff", false)
  Actual: "fn5-_____w"
Expected: "fn5-w7_Dv8O_w78"

C:\Users\Administrator\workspace\mesos\Mesos_CI-build\FLAG\Plain\label\mesos-ec2-windows\mesos\3rdparty\stout\tests\base64_tests.cpp:80
Value of: (base64::decode_url_safe("fn5-w7_Dv8O_w78")).get()
  Actual: "~~~\xC3\xBF\xC3\xBF\xC3\xBF\xC3\xBF"
Expected: "~~~\u00ff\u00ff\u00ff\u00ff"
Which is: "~~~\xFF\xFF\xFF\xFF"