Do not convert text/plain content to JSON

Fixes bug and removes test comments from previous commit.

Resolves #1056

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/catalog/SimpleFunctionRegistryTests.java

@@ -193,7 +193,6 @@ public class SimpleFunctionRegistryTests {
 		assertThat(result).isEqualTo("{\"HELLO\":\"WORLD\"}");
 	}
 
-	// TODO: Once bug is fixed this test (last entry) will fully pass and this COMMENT should be removed
 	@ParameterizedTest
 	@ValueSource(strings = {"[hello", "hello]", "[hello]"})
 	void textContentTypeWithValueWrappedBracketsIsOk(String inputMessagePayloadValue) {