Allow structs as select option values

[ryanwinchester]

Issue

If I try to use a select with struct values, I get an error because of an is_map check, and assumes that it is an option group.

We should be able to pass as values any struct that implements the Phoenix.HTML.Safe protocol.

Changes

• added a struct check
• added a new test assertion that fails without this fix:

  1) test options_for_select/2 simple (Phoenix.HTML.FormTest)
     test/phoenix_html/form_test.exs:262
     ** (Protocol.UndefinedError) protocol Enumerable not implemented for type DateTime (a struct).
     This protocol is implemented for the following type(s): Date.Range, File.Stream, Function,
     GenEvent.Stream, HashDict, HashSet, IO.Stream, List, Map, MapSet, Range, Stream

     Got value:

         ~U[2025-01-01 06:30:00.000000Z]

     code: assert options_for_select([
        {"Open", ~U[2025-01-01 06:30:00.000000Z]},
        {"Close", ~U[2025-01-01 18:30:00.000000Z]}
      ], [
     stacktrace:
       (elixir 1.18.4) lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir 1.18.4) lib/enum.ex:166: Enumerable.reduce/3
       (elixir 1.18.4) lib/enum.ex:4515: Enum.reduce/3
       (phoenix_html 4.2.1) lib/phoenix_html/form.ex:395: Phoenix.HTML.Form.option/4
       (phoenix_html 4.2.1) lib/phoenix_html/form.ex:360: anonymous fn/4 in 
        Phoenix.HTML.Form.escaped_options_for_select/3
       (elixir 1.18.4) lib/enum.ex:2546: Enum."-reduce/3-lists^foldl/2-0-"/3
       (phoenix_html 4.2.1) lib/phoenix_html/form.ex:347: Phoenix.HTML.Form.options_for_select/3
       test/phoenix_html/form_test.exs:306: (test)

[josevalim]

@ryanwinchester oops, can you please run the formatter? 😬

[josevalim]

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