title: Remove headers from openfeign request set by default http client
body: <p>I am calling an API, that blacklists certain HttpHeaders including <code>Content-Length</code> which seems to be preset by the HttpClient underneath spring-openfeign.
  To properly receive an API response, I'd need to remove the <code>Content-Length</code> header.</p>
  
  <p>The following workarounds had been tried:</p>
  
  <ul>
  <li>I tried to set the header to null or an empty String using the available Feign annotations <code>@Headers</code>, <code>@RequestHeaders</code></li>
  <li>I implemented a <code>RequestInterceptor</code> that creates a copy of the available (immutable) header map, deletes the blacklisted header and sets the Map as <code>requestTemplate.headers(newHeaders)</code>. But only new headers can be added and the available ones not modified (seems to be really immutable ;))</li>
  <li>I researched on overriding the used HttpClient but wasn't successful until now.</li>
  </ul>
  
  <p>Experienced errors/ issues:</p>
  
  <p>The API I am calling returns a 400 based on their header schema validation.</p>
  
  <p>Code: </p>
  
  <p>In case any code-snippets are needed, I am happy to provide them but to me the issue does not seem to be related to any code issue as I am not  running into any exceptions.</p>
  
  <p>Thanks in advance!!</p>