HttpHeaders are no longer a MultiValueMap

This change removes the `MultiValueMap` nature of `HttpHeaders`, since
it inherits APIs that do not align well with underlying server
implementations. Notably, methods that allows to iterate over the whole
collection of headers are susceptible to artificially introduced
duplicates when multiple casings are used for a given header, depending
on the underlying implementation.

This change includes a dedicated key set implementation to support
iterator-based removal, and either keeps map method implementations that
are relevant or introduces header-focused methods that have a similar
responsibility (like `hasHeaderValues(String, List)` and
`containsHeaderValue(String, String)`).

In order to nudge users away from using an HttpHeaders as a Map, the
`asSingleValueMap` view is deprecated. In order to offer an escape
hatch to users that do make use of the `MultiValueMap` API, a similar
`asMultiValueMap` view is introduced but is immediately marked as
deprecated.

This change also adds map-like but header-focused assertions to
`HttpHeadersAssert`, since it cannot extend `AbstractMapAssert` anymore.

Closes gh-33913

spring-web/src/testFixtures/java/org/springframework/web/testfixture/http/server/reactive/MockServerHttpRequest.java

@@ -299,9 +299,18 @@ public final class MockServerHttpRequest extends AbstractServerHttpRequest {
 		/**
 		 * Add the given header values.
 		 * @param headers the header values
+		 * @deprecated Use {@link #headers(HttpHeaders)}
 		 */
+		@Deprecated
 		B headers(MultiValueMap<String, String> headers);
 
+		/**
+		 * Add the given header values.
+		 * @param headers the header values
+		 * @since 7.0
+		 */
+		B headers(HttpHeaders headers);
+
 		/**
 		 * Set the list of acceptable {@linkplain MediaType media types}, as
 		 * specified by the {@code Accept} header.
@@ -491,11 +500,18 @@ public final class MockServerHttpRequest extends AbstractServerHttpRequest {
 		}
 
 		@Override
+		@Deprecated
 		public BodyBuilder headers(MultiValueMap<String, String> headers) {
 			this.headers.putAll(headers);
 			return this;
 		}
 
+		@Override
+		public BodyBuilder headers(HttpHeaders headers) {
+			this.headers.putAll(headers);
+			return this;
+		}
+
 		@Override
 		public BodyBuilder accept(MediaType... acceptableMediaTypes) {
 			this.headers.setAccept(Arrays.asList(acceptableMediaTypes));

spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockPart.java

@@ -129,7 +129,7 @@ public class MockPart implements Part {
 
 	@Override
 	public Collection<String> getHeaderNames() {
-		return this.headers.keySet();
+		return this.headers.headerNames();
 	}
 
 	/**