Improve parameter-based content negotiation

Closes gh-36925

spring-web/src/test/java/org/springframework/web/accept/MappingContentNegotiationStrategyTests.java

@@ -49,7 +49,7 @@ class MappingContentNegotiationStrategyTests {
 	}
 
 	@Test
-	void resolveMediaTypesViaMediaTypeFactory() throws Exception {
+	void resolveMediaTypesViaFactory() throws Exception {
 		Map<String, MediaType> mapping = null;
 		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("xml", mapping);
 
@@ -68,6 +68,15 @@ class MappingContentNegotiationStrategyTests {
 				.isInstanceOf(HttpMediaTypeNotAcceptableException.class);
 	}
 
+	@Test
+	void resolveMediaTypesInvalidKey() {
+		Map<String, MediaType> mapping = null;
+		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("not.json", mapping);
+
+		assertThatThrownBy(() -> strategy.resolveMediaTypes(null))
+				.isInstanceOf(HttpMediaTypeNotAcceptableException.class);
+	}
+
 	@Test
 	void resolveMediaTypesNullKey() throws Exception {
 		Map<String, MediaType> mapping = Collections.singletonMap("json", MediaType.APPLICATION_JSON);

spring-web/src/test/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolverTests.java

@@ -75,6 +75,8 @@ class MappingMediaTypeFileExtensionResolverTests {
 		map.put("jSoN", MediaType.APPLICATION_JSON);
 
 		MappingMediaTypeFileExtensionResolver resolver = new MappingMediaTypeFileExtensionResolver(map);
+		map.forEach(resolver::addMapping);
+
 		assertThat(resolver.getAllFileExtensions()).containsExactly("json");
 	}