Fix speed regression in property binding

Update `MapBinder` to only compute values if an entry does not already
exist. Prior to this commit, a binding to `Map<String,List<String>>`
would be expensive since the same entries would be bound many times.

For example, given:

	foo.bar[0]=baz1
	foo.bar[1]=baz1
	foo.bar[2]=baz1

The Map binder would iterate over the properties `bar[0]`, `bar[1]` and
`bar[2]`. Each of these properties resulted in the same actual key of
`bar` which would then be bound to list multiple times.

Fixes gh-10093

spring-boot/src/main/java/org/springframework/boot/context/properties/bind/MapBinder.java

@@ -103,8 +103,8 @@ class MapBinder extends AggregateBinder<Map<Object, Object>> {
 					ConfigurationPropertyName entryName = getEntryName(source, name);
 					Object key = getContext().getConversionService()
 							.convert(getKeyName(entryName), this.keyType);
-					Object value = this.elementBinder.bind(entryName, valueBindable);
-					map.putIfAbsent(key, value);
+					map.computeIfAbsent(key,
+							(k) -> this.elementBinder.bind(entryName, valueBindable));
 				}
 			}
 		}