Guard default conversion service creation

Update ConfigurationPropertiesBindingPostProcessor to ensure that
the defaultConversionService is only created once.

Fixes gh-1822

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

@@ -86,12 +86,10 @@ public class ConfigurationPropertiesBindingPostProcessor implements BeanPostProc
 
 	private ConversionService conversionService;
 
-	private final DefaultConversionService defaultConversionService = new DefaultConversionService();
+	private DefaultConversionService defaultConversionService;
 
 	private BeanFactory beanFactory;
 
-	private final boolean initialized = false;
-
 	private ResourceLoader resourceLoader = new DefaultResourceLoader();
 
 	private Environment environment = new StandardEnvironment();
@@ -341,11 +339,13 @@ public class ConfigurationPropertiesBindingPostProcessor implements BeanPostProc
 	}
 
 	private ConversionService getDefaultConversionService() {
-		if (!this.initialized) {
+		if (this.defaultConversionService == null) {
+			DefaultConversionService conversionService = new DefaultConversionService();
 			for (Converter<?, ?> converter : ((ListableBeanFactory) this.beanFactory)
 					.getBeansOfType(Converter.class, false, false).values()) {
-				this.defaultConversionService.addConverter(converter);
+				conversionService.addConverter(converter);
 			}
+			this.defaultConversionService = conversionService;
 		}
 		return this.defaultConversionService;
 	}