Various MultiValueMap improvements

This commit makes several improvements to MultiValueMap:
- asSingleValueMap offers a single-value view (as opposed to the
  existing toSingleValueMap, which offers a copy)
- fromSingleValue is a static method that adapts a Map<?,?> to the
  MultiValueMap interface
- fromMultiValue is a static method that adapts a Map<?,List<?>> to the
  MultiValueMap interface

Closes gh-32832

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -1774,6 +1774,10 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 		return this.headers.toSingleValueMap();
 	}
 
+	@Override
+	public Map<String, String> asSingleValueMap() {
+		return this.headers.asSingleValueMap();
+	}
 
 	// Map implementation
 

spring-web/src/main/java/org/springframework/http/ReadOnlyHttpHeaders.java

@@ -122,6 +122,11 @@ class ReadOnlyHttpHeaders extends HttpHeaders {
 		return Collections.unmodifiableMap(this.headers.toSingleValueMap());
 	}
 
+	@Override
+	public Map<String, String> asSingleValueMap() {
+		return Collections.unmodifiableMap(this.headers.asSingleValueMap());
+	}
+
 	@Override
 	public Set<String> keySet() {
 		return Collections.unmodifiableSet(this.headers.keySet());

spring-web/src/main/java/org/springframework/web/multipart/support/AbstractMultipartHttpServletRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -102,7 +102,7 @@ public abstract class AbstractMultipartHttpServletRequest extends HttpServletReq
 
 	@Override
 	public Map<String, MultipartFile> getFileMap() {
-		return getMultipartFiles().toSingleValueMap();
+		return getMultipartFiles().asSingleValueMap();
 	}
 
 	@Override