title: Java Spring RestTemplate sets unwanted headers
body: <p>I want to use a service that responds to a rest api. However, when I send a request with the Accept-Charset header set to a long value, that service breaks. An apparently easy solution would be to just set this header explicitly: <code>"Accept-Charset": "utf-8"</code>. However this just does not seem to work:</p>
  
  <pre><code>    String requestBody = "{\"message\": \"I am very frustrated.\"}";
      RestTemplate restTemplate = new RestTemplate();
      HttpHeaders headers = new HttpHeaders();
      ArrayList&lt;Charset&gt; acceptCharset = new ArrayList&lt;&gt;();
      acceptCharset.add(StandardCharsets.UTF_8);
      headers.setAcceptCharset(acceptCharset);
      log.info(headers.toString());
      ResponseEntity&lt;String&gt; res = restTemplate.exchange("http://httpbin.org/post", HttpMethod.POST, new HttpEntity&lt;&gt;(requestBody, headers), String.class);
      String httpbin = res.getBody();
  
      log.info("httpbin result: " + httpbin);
  </code></pre>
  
  <p>This returns this result:</p>
  
  <pre><code>-&gt; {Accept-Charset=[utf-8]}
  
  -&gt; INFO httpbin result: {
    "args": {},
    "data": "{\"message\": \"I am very frustrated.\"}",
    "files": {},
    "form": {},
    "headers": {
      "Accept": "text/plain, application/json, application/*+json, */*",
      "Accept-Charset": "big5, big5-hkscs, cesu-8, euc-jp, euc-kr, gb18030, gb2312, gbk, ibm-thai, ibm00858, ibm01140, ibm01141, ibm01142, ibm01143, ibm01144, ibm01145, ibm01146, ibm01147, ibm01148, ibm01149, ibm037, ibm1026, ibm1047, ibm273, ibm277, ibm278, ibm280, ibm284, ibm285, ibm290, ibm297, ibm420, ibm424, ibm437, ibm500, ibm775, ibm850, ibm852, ibm855, ibm857, ibm860, ibm861, ibm862, ibm863, ibm864, ibm865, ibm866, ibm868, ibm869, ibm870, ibm871, ibm918, iso-2022-cn, iso-2022-jp, iso-2022-jp-2, iso-2022-kr, iso-8859-1, iso-8859-13, iso-8859-15, iso-8859-2, iso-8859-3, iso-8859-4, iso-8859-5, iso-8859-6, iso-8859-7, iso-8859-8, iso-8859-9, jis_x0201, jis_x0212-1990, koi8-r, koi8-u, shift_jis, tis-620, us-ascii, utf-16, utf-16be, utf-16le, utf-32, utf-32be, utf-32le, utf-8, windows-1250, windows-1251, windows-1252, windows-1253, windows-1254, windows-1255, windows-1256, windows-1257, windows-1258, windows-31j, x-big5-hkscs-2001, x-big5-solaris, x-compound_text, x-euc-jp-linux, x-euc-tw, x-eucjp-open, x-ibm1006, x-ibm1025, x-ibm1046, x-ibm1097, x-ibm1098, x-ibm1112, x-ibm1122, x-ibm1123, x-ibm1124, x-ibm1166, x-ibm1364, x-ibm1381, x-ibm1383, x-ibm300, x-ibm33722, x-ibm737, x-ibm833, x-ibm834, x-ibm856, x-ibm874, x-ibm875, x-ibm921, x-ibm922, x-ibm930, x-ibm933, x-ibm935, x-ibm937, x-ibm939, x-ibm942, x-ibm942c, x-ibm943, x-ibm943c, x-ibm948, x-ibm949, x-ibm949c, x-ibm950, x-ibm964, x-ibm970, x-iscii91, x-iso-2022-cn-cns, x-iso-2022-cn-gb, x-iso-8859-11, x-jis0208, x-jisautodetect, x-johab, x-macarabic, x-maccentraleurope, x-maccroatian, x-maccyrillic, x-macdingbat, x-macgreek, x-machebrew, x-maciceland, x-macroman, x-macromania, x-macsymbol, x-macthai, x-macturkish, x-macukraine, x-ms932_0213, x-ms950-hkscs, x-ms950-hkscs-xp, x-mswin-936, x-pck, x-sjis_0213, x-utf-16le-bom, x-utf-32be-bom, x-utf-32le-bom, x-windows-50220, x-windows-50221, x-windows-874, x-windows-949, x-windows-950, x-windows-iso2022jp",
      "Connection": "close",
      "Content-Length": "36",
      "Content-Type": "text/plain;charset=ISO-8859-1",
      "Host": "httpbin.org",
      "User-Agent": "Java/1.8.0_131"
    },
    "json": {
      "message": "I am very frustrated."
    },
    "origin": "###.###.###.###",
    "url": "http://httpbin.org/post"
  }
  </code></pre>
  
  <p>How can I override this default header?</p>