[Elixir] Update Tesla dependency to version 1.0

[yknx4]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
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Description of the PR

• Update Tesla dependency version
• Replace Poison with Jason (it is the new default JSON encoder)
  (Jason doesn't support :as option)
• Use new method to set the headers as per the official migration guide.

closes #2325
cc @mrmstn

[wing328]

@yknx4 thanks for the PR but Shippable CI reports the following errors:

warning: found quoted atom "phone" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:29

warning: found quoted atom "userStatus" but the quotes are not required. Atoms made exclusively of Unicode letters, numbers, underscore, and @ do not require quotes
  lib/openapi_petstore/model/user.ex:30


== Compilation error in file lib/openapi_petstore/model/animal_farm.ex ==
** (ArgumentError) Poison.Encoder is not available, cannot derive Poison.Encoder for OpenapiPetstore.Model.AnimalFarm
    (elixir) lib/protocol.ex:69: Protocol.assert_protocol!/2
    (elixir) lib/protocol.ex:638: Protocol.derive/5
    (stdlib) lists.erl:1338: :lists.foreach/2
    (elixir) lib/protocol.ex:631: Protocol.__derive__/3
    lib/openapi_petstore/model/animal_farm.ex:11: (module)
[ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:1.2.1:exec (compile) on project ElixirPetstoreClientTests: Command execution failed.: Process exited with an error: 1 (Exit value: 1) -> [Help 1]
[ERROR] 

Ref: https://app.shippable.com/github/OpenAPITools/openapi-generator/runs/6398/1/console

Please take a look and let us know if you need any help.

[yknx4]

@wing328 All issues have been addressed :)

[mrmstn]

Hey @yknx4, first of all, thank you very much for your PR

Your changes look pretty good to me, there's only one thing I noticed:
https://github.com/yknx4/openapi-generator/blob/features/update-tesla-dependency-elixir/modules/openapi-generator/src/main/resources/elixir/api.mustache#L61

The new tesla 1.0 will return a tuple ({:ok, %Tesla.Env{}} | {:error, any} ) for Connection.request which wouldn't properly match on RequestBuilder.decode.
( https://github.com/yknx4/openapi-generator/blob/features/update-tesla-dependency-elixir/modules/openapi-generator/src/main/resources/elixir/request_builder.ex.mustache#L119 )

This could be fixes by

• using the request! function instead of request
• or changing the decode function to match on {:ok, %Tesla.Env{} =env} instead of %Tesla.Env{} (which might be the better option since it gives the developer the ability to handle adapter/connection errors without try ... rescue )

Please let me know if I can assist you somehow according to this PR and again, thank you very much for your contribution

[yknx4]

@mrmstn Thank you very much for your feedback, I'll follow the second approach you propose.

[wing328]

If no further feedback on this PR, I'll merge it tomorrow (Wed)