fix err.status invalid lead to uncaughtException

lib/context.js

@@ -115,7 +115,7 @@ var proto = module.exports = {
     if ('ENOENT' == err.code) err.status = 404;
 
     // default to 500
-    err.status = err.status || 500;
+    if ('number' != typeof err.status || !http.STATUS_CODES[err.status]) err.status = 500;
 
     // respond
     var code = http.STATUS_CODES[err.status];

test/context/onerror.js

@@ -50,4 +50,47 @@ describe('ctx.onerror(err)', function(){
       done();
     })
   })
+  describe('when invalid err.status', function(){
+    describe('not number', function(){
+      it('should respond 500', function(done){
+        var app = koa();
+
+        app.use(function *(next){
+          this.body = 'something else';
+          var err = new Error('some error');
+          err.status = 'notnumber';
+          this.throw(err);
+        })
+
+        var server = app.listen();
+
+        request(server)
+        .get('/')
+        .expect(500)
+        .expect('Content-Type', 'text/plain; charset=utf-8')
+        .expect('Internal Server Error', done);
+      })
+    })
+
+    describe('not http status code', function(){
+      it('should respond 500', function(done){
+        var app = koa();
+
+        app.use(function *(next){
+          this.body = 'something else';
+          var err = new Error('some error');
+          err.status = 9999;
+          this.throw(err);
+        })
+
+        var server = app.listen();
+
+        request(server)
+        .get('/')
+        .expect(500)
+        .expect('Content-Type', 'text/plain; charset=utf-8')
+        .expect('Internal Server Error', done);
+      })
+    })
+  })
 })