Reproduce DataEncoder.encode error
Reproduce DataEncoder.encode error

When given a list with a cons operator the DataEncoder has some trouble
encoding the value, instead giving the following error for this test:

```
1) test encode a list a cons operator (Appsignal.Utils.DataEncoderTest)
    test/appsignal/utils/data_encoder_test.exs:101
    ** (FunctionClauseError) no function clause matching in Enum."-each/2-lists^foreach/1-0-"/2

    The following arguments were given to Enum."-each/2-lists^foreach/1-0-"/2:

        # 1
        #Function<1.110451145/1 in Appsignal.Utils.DataEncoder.encode/1>

        # 2
        "foo"

    code: resource = DataEncoder.encode(["" | "foo"])
    stacktrace:
      (elixir) lib/enum.ex:675: Enum."-each/2-lists^foreach/1-0-"/2
      (elixir) lib/enum.ex:675: Enum.each/2
      (appsignal) lib/appsignal/utils/data_encoder.ex:21: Appsignal.Utils.DataEncoder.encode/1
      test/appsignal/utils/data_encoder_test.exs:102: (test)
```

appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir