formFilter append useless character "&" cause body length change

[sunhao1256]

Describe the bug
Prerequisite: use property spring.cloud.gateway.mvc.remove-content-length-request-headers-filter.enabled=false disable RemoveContentLengthFilter.

post form data with query params like "/post?foo=fooquery" , the default rest client jdk client will throw error

[SimpleAsyncTaskExecutor-2] Too many bytes in request body. Expected: 7, got: 8
	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.createResourceAccessException(DefaultRestClient.java:557) ~[spring-web-6.1.5.jar:6.1.5]
	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.exchangeInternal(DefaultRestClient.java:482) ~[spring-web-6.1.5.jar:6.1.5]
	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.exchange(DefaultRestClient.java:449) ~[spring-web-6.1.5.jar:6.1.5]
	at org.springframework.cloud.gateway.server.mvc.handler.RestClientProxyExchange.exchange(RestClientProxyExchange.java:40) ~[classes/:na]

After i dive into code , the following code chunk from form filter .

	static HttpServletRequest getRequestWithBodyFromRequestParameters(HttpServletRequest request) throws IOException {
		ByteArrayOutputStream bos = new ByteArrayOutputStream(1024);
		Writer writer = new OutputStreamWriter(bos, FORM_CHARSET);

		Map<String, String[]> form = request.getParameterMap();
		String queryString = request.getQueryString();
		StringBuffer requestURL = request.getRequestURL();
		if (StringUtils.hasText(queryString)) {
			requestURL.append('?').append(queryString);
		}
		UriComponentsBuilder uriComponentsBuilder = UriComponentsBuilder.fromUriString(requestURL.toString());
		MultiValueMap<String, String> queryParams = uriComponentsBuilder.build().getQueryParams();
		for (Iterator<Map.Entry<String, String[]>> entryIterator = form.entrySet().iterator(); entryIterator
				.hasNext();) {
			Map.Entry<String, String[]> entry = entryIterator.next();
			String name = entry.getKey();
			List<String> values = Arrays.asList(entry.getValue());
			for (Iterator<String> valueIterator = values.iterator(); valueIterator.hasNext();) {
				String value = valueIterator.next();
				List<String> queryValues = queryParams.get(name);
				boolean isQueryParam = queryParams.containsKey(name) && queryValues != null
						&& queryValues.contains(value);
				if (!isQueryParam) {
					writer.write(URLEncoder.encode(name, FORM_CHARSET));
					if (value != null) {
						writer.write('=');
						writer.write(URLEncoder.encode(value, FORM_CHARSET));
						if (valueIterator.hasNext()) {
							writer.write('&');
						}
					}
				}
			}
			if (entryIterator.hasNext()) {
				writer.append('&');
			}
		}
		writer.flush();

		ByteArrayServletInputStream servletInputStream = new ByteArrayServletInputStream(
				new ByteArrayInputStream(bos.toByteArray()));
		return new FormContentRequestWrapper(request, queryParams) {
			@Override
			public ServletInputStream getInputStream() throws IOException {
				return servletInputStream;
			}
		};

The writer if current value isQueryParam means variable isQueryParam return true ,the code still append '&' ,so the body length changed

			if (entryIterator.hasNext()) {
				writer.append('&');
			}

i perfer add a boolen variable to predicate whether should append the character "&"

Sample

@SpringBootTest(properties = {"spring.cloud.gateway.mvc.http-client.type=jdk",
		"spring.cloud.gateway.mvc.remove-content-length-request-headers-filter.enabled=false"},
		webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
public class DisableRemoveContentLengthFilterTests extends ServerMvcIntegrationTests {

	@Test
	void formUrlencodedWorks() {
		LinkedMultiValueMap<String, String> formData = new LinkedMultiValueMap<>();
		formData.add("baz", "bam");

		// @formatter:off
		restClient.post().uri("/post?foo=fooquery").header("test", "formurlencoded")
				.contentType(FORM_URL_ENCODED_CONTENT_TYPE)
				.bodyValue(formData)
				.exchange()
				.expectStatus().isOk()
				.expectBody(Map.class).consumeWith(result -> {
					Map map = result.getResponseBody();
					Map<String, Object> form = getMap(map, "form");
					assertThat(form).containsEntry("baz", "bam");
				});
	}
}

[sunhao1256]

i made a pr to fix the issue #3329

[spencergibb]

Closing in favor of #3329