[MERGE] Only sign 8-bit strings

[Aaron Bentley]

Aaron Bentley has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
Error message has a comma splice.  It should be "You can't sign Unicode 
text; it must be encoded first." or "You can't sign Unicode text.  It 
must be encoded first."

Otherwise, seems reasonable.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45CB52C5.6080209%40arbash-meinel.com%3E