atomic_dbus: fix VulnerableInfo method

As of ef984ed, atomic.get_all_vulnerable_info() sometimes returns a JSON
object instead of a string.

Make it always return an object and amend its documentation.

Closes: #817
Approved by: baude

Atomic/atomic.py

@@ -544,13 +544,12 @@ class Atomic(object):
 
     def get_all_vulnerable_info(self):
         """
-        Will simply read and return the entire /var/lib/atomic/scan_summary.json
-        as a JSON string so it can then be parsed appropriately.
+        Read and parse the /var/lib/atomic/scan_summary.json object.
         """
         try:
             return json.loads(open(os.path.join(self.results, "scan_summary.json"), "r").read())
-        except IOError:
-            return "{}"
+        except (IOError, ValueError):
+            return {}
 
 
     def get_vulnerable_ids(self):

atomic_dbus.py

@@ -272,7 +272,7 @@ class atomic_dbus(slip.dbus.service.Object):
     def VulnerableInfo(self):
         args = self.Args()
         self.atomic.set_args(args)
-        return self.atomic.get_all_vulnerable_info()
+        return json.dumps(self.atomic.get_all_vulnerable_info())
 
     # atomic install section
     # The Install method will install the specified image