Fix corrupted saml2 metadata when special characters are present

Closes gh-13776

Author: Jannick Weisshaupt

saml2/saml2-service-provider/src/main/java/org/springframework/security/saml2/provider/service/web/Saml2MetadataFilter.java

@@ -104,7 +104,7 @@ private void writeMetadataToResponse(HttpServletResponse response, String regist
 		String encodedFileName = URLEncoder.encode(fileName, StandardCharsets.UTF_8.name());
 		String format = "attachment; filename=\"%s\"; filename*=UTF-8''%s";
 		response.setHeader(HttpHeaders.CONTENT_DISPOSITION, String.format(format, fileName, encodedFileName));
-		response.setContentLength(metadata.length());
+		response.setContentLength(metadata.getBytes(StandardCharsets.UTF_8).length);
 		response.setCharacterEncoding(StandardCharsets.UTF_8.name());
 		response.getWriter().write(metadata);
 	}

saml2/saml2-service-provider/src/test/java/org/springframework/security/saml2/provider/service/web/Saml2MetadataFilterTests.java

@@ -166,6 +166,8 @@ public void doFilterWhenCharacterEncodingThenEncodeSpecialCharactersCorrectly()
 		this.filter.doFilter(this.request, this.response, this.chain);
 		assertThat(this.response.getCharacterEncoding()).isEqualTo(StandardCharsets.UTF_8.name());
 		assertThat(this.response.getContentAsString(StandardCharsets.UTF_8)).isEqualTo(generatedMetadata);
+		assertThat(this.response.getContentLength()).isEqualTo(
+				generatedMetadata.getBytes(StandardCharsets.UTF_8).length);
 	}
 
 	@Test