Improve MediaTypeAssert to-string-equality assertions

This commit introduces a `isNotEqualTo(String)` assertion in order to
avoid false negatives when using the default Object-based assertion.

This is a risk since MediaTypeAssert has a `isEqualTo(String)` method
that overrides the base object method and parses the provided String
into a MediaType. Users may thus be tempted to use the reverse assertion
and expect the same parsing behaviour.

This commit also adds tests around the String parsing and the isNotEqual
cases.

Closes gh-32756

spring-test/src/test/java/org/springframework/test/http/MediaTypeAssertTests.java

@@ -43,19 +43,35 @@ class MediaTypeAssertTests {
 	}
 
 	@Test
-	void isEqualWhenSameShouldPass() {
+	void isEqualWhenActualIsNullStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(null).isEqualTo("text/html"))
+				.withMessageContaining("Media type");
+	}
+
+	@Test
+	void isEqualWhenSameStringShouldPass() {
 		assertThat(mediaType("application/json")).isEqualTo("application/json");
 	}
 
 	@Test
-	void isEqualWhenDifferentShouldFail() {
+	void isEqualWhenDifferentStringShouldFail() {
 		assertThatExceptionOfType(AssertionError.class)
 				.isThrownBy(() -> assertThat(mediaType("application/json")).isEqualTo("text/html"))
 				.withMessageContaining("Media type");
 	}
 
 	@Test
-	void isEqualWhenActualIsNullShouldFail() {
+	void isEqualInvalidStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isEqualTo("example of a bad value"))
+				.withMessageContaining("[Media type]")
+				.withMessageEndingWith("To be a valid media type but got:\n" +
+						"  \"Invalid mime type \"example of a bad value\": does not contain '/'\"\n");
+	}
+
+	@Test
+	void isEqualWhenActualIsNullTypeShouldFail() {
 		assertThatExceptionOfType(AssertionError.class)
 				.isThrownBy(() -> assertThat(null).isEqualTo(MediaType.APPLICATION_JSON))
 				.withMessageContaining("Media type");
@@ -73,6 +89,49 @@ class MediaTypeAssertTests {
 				.withMessageContaining("Media type");
 	}
 
+	@Test
+	void isNotEqualWhenActualIsNullStringShouldPass() {
+		assertThat(null).isNotEqualTo("application/json");
+	}
+
+	@Test
+	void isNotEqualWhenDifferentStringShouldPass() {
+		assertThat(mediaType("application/json")).isNotEqualTo("text/html");
+	}
+
+	@Test
+	void isNotEqualWhenSameStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isNotEqualTo("application/json"))
+				.withMessageContaining("Media type");
+	}
+
+	@Test
+	void isNotEqualInvalidStringShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isNotEqualTo("example of a bad value"))
+				.withMessageContaining("[Media type]")
+				.withMessageEndingWith("To be a valid media type but got:\n" +
+						"  \"Invalid mime type \"example of a bad value\": does not contain '/'\"\n");
+	}
+
+	@Test
+	void isNotEqualWhenActualIsNullTypeShouldPass() {
+		assertThat(null).isNotEqualTo(MediaType.APPLICATION_JSON);
+	}
+
+	@Test
+	void isNotEqualWhenDifferentTypeShouldPass() {
+		assertThat(mediaType("application/json")).isNotEqualTo(MediaType.TEXT_HTML);
+	}
+
+	@Test
+	void isNotEqualWhenSameTypeShouldFail() {
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertThat(mediaType("application/json")).isNotEqualTo(MediaType.APPLICATION_JSON))
+				.withMessageContaining("Media type");
+	}
+
 	@Test
 	void isCompatibleWhenSameShouldPass() {
 		assertThat(mediaType("application/json")).isCompatibleWith("application/json");