Sending form feed characters (\f) does not work

[eiriksm]

Hey. Not sure if this is a more general question or if I should report it here, but since this can be fixed in the php library (which i need) I am going to ask it here :)

Anyway, simple reproducible code:

<?php
require 'vendor/autoload.php';

use SparkPost\SparkPost;
use GuzzleHttp\Client;
use Http\Adapter\Guzzle6\Client as GuzzleAdapter;

$my_mail = '[email protected]';

$httpClient = new GuzzleAdapter(new Client());
$sparky = new SparkPost($httpClient, ['key'=>'xxx']);
$promise = $sparky->transmissions->post([
  'content' => [
    'from' => [
      'name' => 'test',
      'email' => $my_mail,
    ],
    'subject' => 'First Mailing From PHP',
    'html' => "\f",
    'text' => 'Congratulations, {{name}}!! You just sent your very first mailing!',
  ],
  'substitution_data' => ['name' => 'YOUR_FIRST_NAME'],
  'recipients' => [
    [
      'address' => [
        'name' => 'YOUR_NAME',
        'email' => $my_mail,
      ],
    ],
  ],
]);

try {
  $response = $promise->wait();
  echo "SUCCESS!\n";
  echo $response->getStatusCode()."\n";
  print_r($response->getBody())."\n";
} catch (\Exception $e) {
  echo "FAIL!\n";
  echo $e->getCode()."\n";
  echo $e->getMessage()."\n";
}

Expected result is to be sent successfully. Output is:

FAIL!
422
{ "errors": [ { "message": "invalid data format\/type", "description": "Problems parsing request as json", "code": "1300" } ] }

The issue also persists if you have the character somewhere inside your string too. Or just as any part of what ends up in the JSON, for example the sender or subject.