Fix encoding of nested parameters in multipart requests

[ob-stripe]

r? @brandur-stripe @remi-stripe
cc @stripe/api-libraries

For multipart requests, we don't encode the parameters ourselves and instead pass them directly to curl:

stripe-php/lib/HttpClient/CurlClient.php

Line 194 in d14f02d

$opts[CURLOPT_POSTFIELDS] = $hasFile ? $params : Util\Util::encodeParameters($params);

However, curl doesn't know how to encode arrays and encodes them as Array regardless of their contents. For instance this request:

$file = \Stripe\File::create([
  "file" => new \CurlFile("/Users/ob/test.txt"),
  "purpose" => "identity_document",
  "file_link_data" => ["create" => true],
]);

would be encoded as:

--------------------------9f9f472a73222896
Content-Disposition: form-data; name="file"; filename="/Users/ob/test.txt"
Content-Type: application/octet-stream

hello

--------------------------9f9f472a73222896
Content-Disposition: form-data; name="purpose"

identity_document
--------------------------9f9f472a73222896
Content-Disposition: form-data; name="file_link_data"

Array
--------------------------9f9f472a73222896--

which of course causes the request to fail.

This PR fixes this by manually flattening the parameters before passing them to curl. It's unfortunately difficult to write tests for this because we can't mock curl, but I've verified that requests are encoded correctly even with deeply nested parameters. E.g. with this patch, the following request:

$file = \Stripe\File::create([
  "file" => new \CurlFile("/Users/ob/test.txt"),
  "purpose" => "identity_document",
  "file_link_data" => [
    "create" => true,
    "expires_at" => 1234567890,
    "metadata" => [
      "key1" => "value1",
      "key2" => "value2",
    ],
  ],
]);

is encoded as:

--------------------------344ee1cb94395191
Content-Disposition: form-data; name="file"; filename="/Users/ob/test.txt"
Content-Type: application/octet-stream

hello

--------------------------344ee1cb94395191
Content-Disposition: form-data; name="purpose"

identity_document
--------------------------344ee1cb94395191
Content-Disposition: form-data; name="file_link_data[create]"

true
--------------------------344ee1cb94395191
Content-Disposition: form-data; name="file_link_data[expires_at]"

1234567890
--------------------------344ee1cb94395191
Content-Disposition: form-data; name="file_link_data[metadata][key1]"

value1
--------------------------344ee1cb94395191
Content-Disposition: form-data; name="file_link_data[metadata][key2]"

value2
--------------------------344ee1cb94395191--

[ob-stripe]

Of course my elegant one-liner doesn't work on PHP 5.4 😢

[remi-stripe]

This looks fine to me especially as you explicitly tested manually but deferring to @brandur-stripe to approve

[brandur-stripe]

Nice fix OB! Looks good to me as well.

[ob-stripe]

Thanks both!

[ob-stripe]

Released as 6.31.4.