### Question:
MimeHeadersSupportFilter extends HttpFilter { @Override protected void doFilter(HttpServletRequest req, HttpServletResponse res, FilterChain chain) throws IOException, ServletException { HttpServletRequestWrapper request = new MimeHeaderRequestWrapper(req); chain.doFilter(request, res); }    }### Answer:
@Test void doFilter() throws Exception { String encodedUmlauts = "=?UTF-8?B?w6Qgw7Ygw7w=?="; ArgumentCaptor<RequestHeaders> headersCaptor = ArgumentCaptor.forClass(RequestHeaders.class); when(accountInformationService.createConsent(headersCaptor.capture(), any(), any())) .thenThrow(new AspspRegistrationNotFoundException("")); mockMvc.perform(post(ConsentController.CONSENTS) .header(RequestHeaders.PSU_ID, encodedUmlauts) .contentType(APPLICATION_JSON_VALUE) .content("{}")) .andExpect(status().is(HttpStatus.BAD_REQUEST.value())) .andReturn(); RequestHeaders headers = headersCaptor.getValue(); assertThat(headers.get(RequestHeaders.PSU_ID)).contains("ä ö ü"); }