[bug] Don't render the inner_block assign when building element attributes

[mhanberg]

Description can be seen here: https://elixirforum.com/t/temple-html-dsl-for-elixir-and-phoenix/23940/29?u=mhanberg

this should reject the inner_block key and not render it

assigns = [class: "foo", inner_block: fn x -> "could be any type tho" end]

div assigns do
  # ...
end

<div class="foo">
  <!-- ... -->
</div>

Currently, this will attempt to render it, but a function does not implement the phoenix Safe protocol, so it fails.

We can also potentially emit a warning if you try to pass types that cannot be rendered, so that the user is aware of why it's not "working".

[mhanberg]

This actually works as expected and is correct.