Swagger definitions now have type set to object.

While the spec does not seem to require this, the Swagger editor at
http://editor.swagger.io/ gets confused and does not display the
'Try this operation' UI correctly.

Many of the examples found around also specify `"type": "object"`,
so this *should* be the right thing to do.

protoc-gen-swagger/genswagger/template.go

@@ -51,6 +51,7 @@ func findNestedMessagesAndEnumerations(message *descriptor.Message, reg *descrip
 func renderMessagesAsDefinition(messages messageMap, d swaggerDefinitionsObject, reg *descriptor.Registry) {
 	for _, msg := range messages {
 		object := swaggerSchemaObject{
+			Type: "object",
 			Properties: map[string]swaggerSchemaObject{},
 		}
 		for _, field := range msg.Fields {