software-properties-gtk crash if accentuated characters in apt-key list

software-properties-gtk crashes badly if one of the apt keys listed in apt-key list contains an accentuated char.

Here I have retraced it to having a key name containing a "ç".

$ /etc/apt$ software-properties-gtk

Traceback (most recent call last):
  File "/usr/bin/software-properties-gtk", line 101, in <module>
    app = SoftwarePropertiesGtk(datadir=options.data_dir, options=options, file=file)
  File "/usr/lib/python3/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 169, in __init__
    self.show_keys()
  File "/usr/lib/python3/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py", line 846, in show_keys
    for key in self.apt_key.list():
  File "/usr/lib/python3/dist-packages/softwareproperties/AptAuth.py", line 75, in list
    for line in p:
  File "/usr/lib/python3.4/codecs.py", line 313, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 2553: invalid continuation byte

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: software-properties-gtk 0.92.37.2
ProcVersionSignature: Ubuntu 3.13.0-39.66-generic 3.13.11.8
Uname: Linux 3.13.0-39-generic i686
NonfreeKernelModules: nvidia
ApportVersion: 2.14.1-0ubuntu3.5
Architecture: i386
CurrentDesktop: Unity
Date: Sat Nov 22 15:34:45 2014
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: Upgraded to trusty on 2014-04-20 (215 days ago)