Enforce custom string converter in HttpMessageConverters

Prior to this commit, configuring a custom `StringHttpMessageConverter`
would be overwritten when the `registerDefaults()` option is enabled.

Fixes gh-35563

spring-web/src/main/java/org/springframework/http/converter/DefaultHttpMessageConverters.java

@@ -247,8 +247,10 @@ class DefaultHttpMessageConverters implements HttpMessageConverters {
 
 		void detectMessageConverters() {
 			this.byteArrayMessageConverter = new ByteArrayHttpMessageConverter();
-			this.stringMessageConverter = new StringHttpMessageConverter();
 
+			if (this.stringMessageConverter == null) {
+				this.stringMessageConverter = new StringHttpMessageConverter();
+			}
 			if (this.jsonMessageConverter == null) {
 				if (JACKSON_PRESENT) {
 					this.jsonMessageConverter = new JacksonJsonHttpMessageConverter();

spring-web/src/test/java/org/springframework/http/converter/DefaultHttpMessageConvertersTests.java

@@ -177,6 +177,16 @@ class DefaultHttpMessageConvertersTests {
 			assertThat(customConverter).isEqualTo(jacksonConverter);
 		}
 
+		@Test
+		void shouldOverrideStringConverters() {
+			var stringConverter = new StringHttpMessageConverter();
+			var converters = HttpMessageConverters.forClient().registerDefaults()
+					.stringMessageConverter(stringConverter).build();
+
+			var actualConverter = findMessageConverter(StringHttpMessageConverter.class, converters);
+			assertThat(actualConverter).isEqualTo(stringConverter);
+		}
+
 		@Test
 		void shouldConfigureConverter() {
 			var customConverter = new CustomHttpMessageConverter();
@@ -271,6 +281,17 @@ class DefaultHttpMessageConvertersTests {
 			assertThat(customConverter).isEqualTo(jacksonConverter);
 		}
 
+
+		@Test
+		void shouldOverrideStringConverters() {
+			var stringConverter = new StringHttpMessageConverter();
+			var converters = HttpMessageConverters.forServer().registerDefaults()
+					.stringMessageConverter(stringConverter).build();
+
+			var actualConverter = findMessageConverter(StringHttpMessageConverter.class, converters);
+			assertThat(actualConverter).isEqualTo(stringConverter);
+		}
+
 		@Test
 		void shouldConfigureConverter() {
 			var customConverter = new CustomHttpMessageConverter();