fix: camelize when prefix is camelized already

[rodrigues]

This PR fixes a problem when payload relationship is already camelized:

In the POST, when you receive as payload:

%{
  type: "example-resource",
  attributes: %{...},
  relationships: %{
    generalManagerProfile: %{data: %{id: manager_id, type: "profiles"}}
  }
}

params will resolve to:

%{"generalmanagerprofileId" => 139981}

with this fix, it resolves correctly to generalManagerProfileId, and with the UnderscoreParameters plug, becomes general_manager_profile_id.

[mattpolzin]

This looks like a good idea to me, but I think it could be argued to regress on something that admittedly is not strictly defined behavior: ThisThing used to be transformed into something starting with a lower case t but won't anymore, right?

[rodrigues]

good point @mattpolzin, thanks, I've pushed a change to address this scenario too:

iex> camelize("PascalLambda_id")
      "pascalLambdaId"