PostedFiles lost after trying to find nonexistent Param

[mat285]

What happened:

Calling web.Ctx.PostedFiles or webutil.PostedFiles(web.Ctx.Request) returns no files if web.Ctx.Param("param") is called beforehand with a missing parameter.

What you expected to happen:

PostedFiles should return the files from the request even if web.Ctx.Param is called with a missing parameter

How to reproduce it (as minimally and precisely as possible):

app := web.New(web.OptBindAddr(":8080"), web.OptLog(logger.All()))

app.POST("/file", func(r *web.Ctx) web.Result {
	r.Param("non-existent")
	files, err := webutil.PostedFiles(r.Request)
	if err != nil {
		return web.Text.BadRequest(err)
	}
	fmt.Println("num files", len(files))
	return web.Text.OK()
})```


**Anything else we need to know?**:

This happens because the request gets consumed by the `r.Param` searching for all of the possible parameters

**Environment**:
- go-sdk version: master
- go version (e.g `go version`): go1.11.4 darwin/amd64
- OS (e.g: `cat /etc/os-release`): macOS 10.13.4
- Others: