fix variable and type mismatch errors

Hello, Shapeways

I found when run `go test`

```bash
$ go test github.com/Shapeways/go-shapeways/shapeways
# github.com/Shapeways/go-shapeways/shapeways
shapeways/shapeways.go:157: undefined: CartData
shapeways/shapeways.go:161: undefined: CartData
shapeways/shapeways.go:177: cannot use Page (type int) as type string in function argument
shapeways/shapeways.go:208: undefined: Modelid
shapeways/shapeways.go:213: undefined: Modelid
shapeways/shapeways.go:218: undefined: Modelid
shapeways/shapeways.go:222: undefined: boolean
shapeways/shapeways.go:225: cannot use 1 (type int) as type string in function argument
shapeways/shapeways.go:227: cannot use 0 (type int) as type string in function argument
FAIL    github.com/Shapeways/go-shapeways/shapeways [build failed]
```

I fix this, please review and comment the code.

Thank you