Decode i18n values from UTF-8 in ClamiAuthFilter.

Some of the attributes we extract in ClaimAuthFilter are
internationalized strings (i18n). We expect these will be encoded in
UTF-8 therefore we must decode them from UTF-8. There are extensive
comments in the code explaining the issues.

Change-Id: I7b4a437432c0e6d3b6c24f552a8886f54aabb1b5
Signed-off-by: John Dennis <jdennis@redhat.com>