Support Map in FormHttpMessageConverter

This commit changes the FormHttpMessageConverter from a
HttpMessageConverter<MultiValueMap<String, ?>> to a
HttpMessageConverter<Map<String, ?>>, so that normal, single-value maps
can also be used as form representation, both for reading and writing.

Closes gh-32826

spring-test/src/main/java/org/springframework/test/web/client/match/ContentRequestMatchers.java

@@ -174,12 +174,13 @@ public class ContentRequestMatchers {
 		return formData(multiValueMap, false);
 	}
 
+	@SuppressWarnings("unchecked")
 	private RequestMatcher formData(MultiValueMap<String, String> expectedMap, boolean containsExactly) {
 		return request -> {
 			MockClientHttpRequest mockRequest = (MockClientHttpRequest) request;
 			MockHttpInputMessage message = new MockHttpInputMessage(mockRequest.getBodyAsBytes());
 			message.getHeaders().putAll(mockRequest.getHeaders());
-			MultiValueMap<String, String> actualMap = new FormHttpMessageConverter().read(null, message);
+			MultiValueMap<String, String> actualMap = (MultiValueMap<String, String>) new FormHttpMessageConverter().read(null, message);
 			if (containsExactly) {
 				assertEquals("Form data", expectedMap, actualMap);
 			}

spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java

@@ -899,6 +899,7 @@ public class MockHttpServletRequestBuilder
 		}
 	}
 
+	@SuppressWarnings("unchecked")
 	private MultiValueMap<String, String> parseFormData(MediaType mediaType) {
 		HttpInputMessage message = new HttpInputMessage() {
 			@Override
@@ -914,7 +915,7 @@ public class MockHttpServletRequestBuilder
 		};
 
 		try {
-			return new FormHttpMessageConverter().read(null, message);
+			return (MultiValueMap<String, String>) new FormHttpMessageConverter().read(null, message);
 		}
 		catch (IOException ex) {
 			throw new IllegalStateException("Failed to parse form data in request body", ex);