Testing POST ActionDispatch::Http::Parameters::ParseError: 765 : <p>It says there's an unexpected token in my params.</p>

<p><code>"ActionDispatch::Http::Parameters::ParseError: 765: unexpected token at 'conversation_identifier[participant_list][]=2&amp;conversation_identifier[participant_list][]=1"</code></p>

<p>A version of the test with magic numbers for clarity:</p>

<pre><code>let(:headers) do
  { 'HTTP_CURRENT_USER_ID' =&gt; 2,
    'Content-Type'         =&gt; 'application/json' }
end
let(:params) { { conversation_identifier: { participant_list: [1, 2] } }

it 'is getting testy' do
  post resource_url, params: params, headers: headers
  assert_equal 201, response.status
end
</code></pre>

<p>Now here's what's weird. It has no trouble parsing those params if I give it no headers.</p>
