Fix bug in SimpleMethodMetadataReadingVisitor.Source.toString()

Prior to this commit, the toString() implementation did not separate
method argument types with a comma or any form of separator, leading
to results such as:

    org.example.MyClass.myMethod(java.lang.Stringjava.lang.Integer)

instead of:

    org.example.MyClass.myMethod(java.lang.String,java.lang.Integer)

Closes spring-projectsgh-27095

spring-core/src/main/java/org/springframework/core/type/classreading/SimpleMethodMetadataReadingVisitor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,6 +32,7 @@
  * ASM method visitor that creates {@link SimpleMethodMetadata}.
  *
  * @author Phillip Webb
+ * @author Sam Brannen
  * @since 5.2
  */
 final class SimpleMethodMetadataReadingVisitor extends MethodVisitor {
@@ -144,14 +145,17 @@ public String toString() {
 			if (value == null) {
 				StringBuilder builder = new StringBuilder();
 				builder.append(this.declaringClassName);
-				builder.append(".");
+				builder.append('.');
 				builder.append(this.methodName);
 				Type[] argumentTypes = Type.getArgumentTypes(this.descriptor);
-				builder.append("(");
-				for (Type type : argumentTypes) {
-					builder.append(type.getClassName());
+				builder.append('(');
+				for (int i = 0; i < argumentTypes.length; i++) {
+					if (i != 0) {
+						builder.append(',');
+					}
+					builder.append(argumentTypes[i].getClassName());
 				}
-				builder.append(")");
+				builder.append(')');
 				value = builder.toString();
 				this.toStringValue = value;
 			}