allow form parameters and missing body is a string

see https://github.com/krakenjs/swaggerize-express/issues/18

- req.body defaults to {} when no body is present. Results in 'invalid type'. A {} is now converted to '' (empty string)
- parameter types not a path or query where all mapped to req.body. In case of form, it now reads from the body object (eg when using `app.use(bodyParser.urlencoded({ extended: true }))`)