Keyword arugments in `@model` are not treated as data

In the MEW below, `x_inf` is not treated as data.

```julia
using Turing

@model mwe(x0=1.0) = begin
    m ~ Normal(0, 1)
    x0 ~ Normal(m, 1)
end

sample(mwe(), HMC(0.1, 2), 100)
```

However, this works

```julia
sample(mwe(1.0), HMC(0.1, 2), 100)
```