Fix username handling on Windows

We forcibly encoded the unicode string to UTF-8 right after retrieval
which broke the encoding later on.

This patch fixes this issue.

Change-Id: I65b33e4f38f801f1770cf94a5ed36234c2fae90d
Bug-Url: https://bugzilla.redhat.com/1010201
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>