I'm not following your issue here. If you're doing a var_dump($headerArray) then you should be seeing something similar to this:

array(1) {
  [0] =>
  string(16) "x-api-key: 12345"
}

This is fully expected. The keys in this array aren't used by curl (or any of the other transports).

The line you highlighted takes the $headers associative array you injected into the Http::post() method and standardizes the header into a string notation (and in newer code also supports nested arrays). Essentially, you are getting this change:

// Passing this...
$headers = array(
    'x-api-key' => '12345',
);

// ... becomes this
$headers = array(
    'x-api-key: 12345',
);

Hi,
when I do a var_dump($headers) (what I pass in the ->post) and a var_dump($headersArray) (what is set in the CURLOPT_HTTPHEADER) in Curltransport I get:

/var/www/html/project/equities/libraries/src/Http/Transport/CurlTransport.php:126:
array (size=6)
  0 => string 'Date: Wed, 10 Apr 2019 13:53:38 GMT' (length=35)
  1 => string 'Authorization: Auth [secret]' (length=65)
  2 => string 'Content-Md5: [secret]==' (length=37)
  3 => string 'Content-Type: application/json' (length=30)
  4 => string 'X-Mitter-Application-Access-Key: [secret]' (length=49)
  5 => string 'Nonce: [secret]' (length=57)

/var/www/html/project/equities/libraries/src/Http/Transport/CurlTransport.php:131:
array (size=6)
  0 => string '0: Date: Wed, 10 Apr 2019 13:53:38 GMT' (length=38)
  1 => string '1: Authorization: Auth [secret]' (length=68)
  2 => string '2: Content-Md5: [secret]==' (length=40)
  3 => string '3: Content-Type: application/json' (length=33)
  4 => string '4: X-Mitter-Application-Access-Key: [secret]' (length=52)
  5 => string '5: Nonce: [secret]' (length=60)

Also found this: https://tools.ietf.org/html/rfc2616#section-4.2

Each header field consists of a name followed by a colon (":") and the field value.

What happens here is that the header field consists of a number followed by a colon followed by the name folowed by a colon forllowed by the field value.

Your issue is you need to pass an associative array, not an already formatted array.

It looks like you're using this for your $headers array:

$headers = array(
    'Date: Wed, 10 Apr 2019 13:53:38 GMT',
    'Authorization: Auth [secret]',
    'Content-Md5: [secret]==',
    'Content-Type: application/json',
    'X-Mitter-Application-Access-Key: [secret]',
    'Nonce: [secret]',
);

This is incorrect. It needs to be an associative array where the key is the header name and the value is the header value.

$headers = array(
    'Date' => 'Wed, 10 Apr 2019 13:53:38 GMT',
    'Authorization' => 'Auth [secret]',
    'Content-Md5' => '[secret]==',
    'Content-Type' => 'application/json',
    'X-Mitter-Application-Access-Key' => '[secret]',
    'Nonce' => '[secret]',
);

owww @mbabker , you just made my day :) that is indeed the what happened, shame on me for copying from the API providers documentation :(