Fix regression in BodyInserters with multipart data

The issue was introduced very recently with commit #7035ee but never
released.

Issue: SPR-16350

Author: rstoyanchev

spring-webflux/src/main/java/org/springframework/web/reactive/function/BodyInserters.java

@@ -479,7 +479,9 @@ public MultipartInserter with(String key, @Nullable Object value) {
 		public MultipartInserter with(MultiValueMap<String, Object> values) {
 			Assert.notNull(values, "'values' must not be null");
 			for (Map.Entry<String, List<Object>> entry : values.entrySet()) {
-				this.builder.part(entry.getKey(), entry.getValue());
+				for (Object value : entry.getValue()) {
+					this.builder.part(entry.getKey(), value);
+				}
 			}
 			return this;
 		}

spring-webflux/src/test/java/org/springframework/web/reactive/function/BodyInsertersTests.java

@@ -21,6 +21,7 @@
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -64,6 +65,7 @@
 import org.springframework.util.MultiValueMap;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 import static org.springframework.http.codec.json.Jackson2CodecSupport.JSON_VIEW_HINT;
 
@@ -320,6 +322,35 @@ public void fromMultipartData() throws Exception {
 
 	}
 
+	@Test // SPR-16350
+	public void fromMultipartDataWithMultipleValues() {
+		MultiValueMap<String, Object> map = new LinkedMultiValueMap<>();
+		map.put("name", Arrays.asList("value1", "value2"));
+		BodyInserters.FormInserter<Object> inserter = BodyInserters.fromMultipartData(map);
+
+		MockClientHttpRequest request = new MockClientHttpRequest(HttpMethod.GET, URI.create("http://example.com"));
+		Mono<Void> result = inserter.insert(request, this.context);
+		StepVerifier.create(result).expectComplete().verify();
+
+		StepVerifier.create(request.getBody().reduce(DataBuffer::write))
+				.consumeNextWith(dataBuffer -> {
+					byte[] resultBytes = new byte[dataBuffer.readableByteCount()];
+					dataBuffer.read(resultBytes);
+					DataBufferUtils.release(dataBuffer);
+					String content = new String(resultBytes, StandardCharsets.UTF_8);
+					assertThat(content, containsString("Content-Disposition: form-data; name=\"name\"\r\n" +
+							"Content-Type: text/plain;charset=UTF-8\r\n" +
+							"\r\n" +
+							"value1"));
+					assertThat(content, containsString("Content-Disposition: form-data; name=\"name\"\r\n" +
+							"Content-Type: text/plain;charset=UTF-8\r\n" +
+							"\r\n" +
+							"value2"));
+				})
+				.expectComplete()
+				.verify();
+	}
+
 	@Test
 	public void ofDataBuffers() throws Exception {
 		DefaultDataBufferFactory factory = new DefaultDataBufferFactory();