add redirect and update spec.jl

Add redirect(https://github.com/JuliaWeb/Morsel.jl/issues/20) .

Some test code as below

```

using Morsel
app = Morsel.app()

get(app, "/") do req, res
    "root"
end

get(app, "/redirect302") do req, res
  redirect(res, "/")
end

get(app, "/redirect301") do req, res
  redirect(res, "/", 301)
end

start(app, 8000)


```