Ensure log.info({err: err}) results in a "msg" value, just like log.info(err)

Fixes #426

CHANGES.md

@@ -8,7 +8,8 @@ Known issues:
 
 ## 1.8.2 (not yet released)
 
-(nothing yet)
+- [issue #426] Ensure `log.info({err: err})` results in a "msg" value, just
+  like `log.info(err)`.
 
 
 ## 1.8.1

lib/bunyan.js

@@ -980,7 +980,13 @@ function mkLogEmitter(minLevel) {
                 msgArgs[0] = util.inspect(msgArgs[0]);
             } else {  // `log.<level>(fields, msg, ...)`
                 fields = args[0];
-                msgArgs = Array.prototype.slice.call(args, 1);
+                if (args.length === 1 && fields.err
+                    && fields.err instanceof Error)
+                {
+                    msgArgs = [fields.err.message];
+                } else {
+                    msgArgs = Array.prototype.slice.call(args, 1);
+                }
             }
 
             // Build up the record object.