add overriding to application/json on ctx.body=object

since it cant be anything else, but if you have middleware that transforms
the object to xml or something then you could set ctx.type=

lib/context.js

@@ -187,7 +187,7 @@ module.exports = {
     }
 
     // json
-    if (setType) this.type = 'json';
+    this.type = 'json';
   },
 
   /**

test/context.js

@@ -30,11 +30,23 @@ describe('ctx.body=', function(){
       assert('image/png' == ctx.responseHeader['content-type']);
     })
 
+    describe('when body is an object', function(){
+      it('should override as json', function(){
+        var ctx = context();
+
+        ctx.body = '<em>hey</em>';
+        assert('text/html; charset=utf-8' == ctx.responseHeader['content-type']);
+        
+        ctx.body = { foo: 'bar' };
+        assert('application/json' == ctx.responseHeader['content-type']);
+      })
+    })
+
     it('should override length', function(){
       var ctx = context();
       ctx.type = 'html';
       ctx.body = 'something';
-      assert.equal(ctx.responseLength, 9);
+      ctx.responseLength.should.equal(9);
     })
   })