INT-3474 Suppress DEBUG StackTrace in MIMHelper

JIRA: https://jira.spring.io/browse/INT-3474

When the MethodInvokingMessageHandler skips ineligible methods while
searching it emits a stack trace under DEBUG logging. This is not
needed, a simple log message is enough to convey the information.

Some POJOs gain ineligible methods when wrapped in a proxy so this
is no fault of the developer.

Other reasons for skipping methods (improperly set annotations etc)
continue to emit a stack trace.

INT-3474: Polishing

Rethrow `IneligibleMethodException` as `IllegalArgumentException` in case of a single `method` for processor

spring-integration-core/src/test/java/org/springframework/integration/handler/MethodInvokingMessageProcessorTests.java

@@ -506,6 +506,16 @@ public class MethodInvokingMessageProcessorTests {
 		assertEquals("FOO", helper.process(new GenericMessage<Object>(targetObject)));
 	}
 
+	@Test
+	public void testIneligible() {
+		IneligibleMethodBean bean = new IneligibleMethodBean();
+		MethodInvokingMessageProcessor processor = new MethodInvokingMessageProcessor(bean, "foo");
+		processor.processMessage(MessageBuilder.withPayload("true").build());
+		assertNotNull(bean.lastArg);
+		assertEquals(String.class, bean.lastArg.getClass());
+		assertEquals("true", bean.lastArg);
+	}
+
 	private static class ExceptionCauseMatcher extends TypeSafeMatcher<Exception> {
 		private Throwable cause;
 
@@ -683,4 +693,20 @@ public class MethodInvokingMessageProcessorTests {
 
 	}
 
+	private static class IneligibleMethodBean {
+
+		private volatile Object lastArg = null;
+
+		@SuppressWarnings("unused")
+		public void foo(String s) {
+			this.lastArg = s;
+		}
+
+		@SuppressWarnings("unused")
+		public void foo(String s, int i) {
+			throw new RuntimeException("expected ineligible");
+		}
+
+	}
+
 }