Fix Elixir 1.17 warnings

[FlXlT]

Resolves the following warnings that arise in Elixir 1.17

warning: using map.field notation (without parentheses) to invoke function Maru.EntityTest.ReturnOnlyWantedFieldsWithBeforeSerializeTest.__exposures__() is deprecated, you must add parentheses instead: remote.function()
  (maru_entity 0.2.3) lib/maru/entity/runtime.ex:358: Maru.Entity.Runtime.do_serialize/2

[falood]

thanks

[FlXlT]

thank you!