fix S3 #delete_multiple_objects for UTF-8 names

[alepore]

Hello!
I was debugging backup/backup#670 and i found out that fog-aws' #delete_multiple_objects does not work with UTF-8 file names.

The problem is in the 'Content-Length' header, String#length is not the right method to find UTF-8 strings size:

> "fØg_öbjèct".length
 => 10
> "fØg_öbjèct".bytesize
 => 13

Please review the test, it not very clear to me if that's enough (#311)

[lanej]

@alepore good catch.

[geemus]

Thanks!