Add "wrapper" type methods to FunctionInspector

These can be used to more reliably discover whether the user
has declared a function with flux types or "bare" POJOs. They
then pave the way to supporting single valued types in a special
way.

Also consolidate and simplify the logic in FunctionInspector

spring-cloud-function-core/src/main/java/org/springframework/cloud/function/support/FunctionUtils.java

@@ -60,7 +60,10 @@ public abstract class FunctionUtils {
 		}
 		String[] types = getParameterizedTypeNames(supplier, Supplier.class);
 		if (ObjectUtils.isEmpty(types)) {
-			return true;
+			// Assume if there is no generic information then the function is not
+			// expecting a flux. N.B. this isn't very accurate. It is better to use a
+			// FunctionInspector if one is available.
+			return false;
 		}
 		return (types[0].startsWith(FLUX_CLASS_NAME));
 	}