fix: use map update to avoid struct type warning in add_namespace/3

[l1x]

Elixir 1.19+ type checker warns when using struct update syntax with a dynamic type. Since the function head already pattern matches on %SweetXpath{}, we can safely use map update syntax instead.

[Shakadak]

Hello, the warning was removed before releasing elixir 1.19, they might come back in 1.20 though.
Considering this uncertainty, I would rather hold off removing assertions in the code that could help for tools like dialyzer and language servers when using older versions of Elixir (and we do have a lot of our projects at work in this situation).

[hooch]

Still seeing the above warning in current Elixir 1.19.5

The following fix is required in 1.20.0-rc.3 to resolve for the warning below:

lib/sweet_xml.ex
╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌
 934
 935    defp split_last_whitespace(size, bin) do
 936      case bin do
 937 -      <<_::binary-size(size), h>> <> tail when h == ?\s or h == ?\n or h == ?\r or h == ?\t ->
 937 +      <<_::binary-size(^size), h>> <> tail when h == ?\s or h == ?\n or h == ?\r or h == ?\t ->
 938          {head, _} = :erlang.split_binary(bin, size + 1)

 ==> sweet_xml
     Compiling 2 files (.ex)
          warning: the variable "size" is accessed inside size(...) of a bitstring but it was defined outside of the match. You
      must precede it with the pin operator
          │
      937 │       <<_::binary-size(size), h>> <> tail when h == ?\s or h == ?\n or h == ?\r or h == ?\t ->
          │                        ~
          │
          └─ lib/sweet_xml.ex:937:24: SweetXml.split_last_whitespace/2