Ignore unresolvable placeholders during property binding

Closes gh-13122

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

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -51,7 +51,7 @@ public class PropertySourcesPlaceholdersResolver implements PlaceholdersResolver
 		this.helper = (helper != null ? helper
 				: new PropertyPlaceholderHelper(SystemPropertyUtils.PLACEHOLDER_PREFIX,
 						SystemPropertyUtils.PLACEHOLDER_SUFFIX,
-						SystemPropertyUtils.VALUE_SEPARATOR, false));
+						SystemPropertyUtils.VALUE_SEPARATOR, true));
 	}
 
 	@Override

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/BinderTests.java

@@ -30,7 +30,6 @@ import org.assertj.core.matcher.AssertionMatcher;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
-import org.junit.internal.matchers.ThrowableMessageMatcher;
 import org.junit.rules.ExpectedException;
 import org.mockito.Answers;
 import org.mockito.InOrder;
@@ -53,7 +52,6 @@ import org.springframework.validation.annotation.Validated;
 import org.springframework.validation.beanvalidation.SpringValidatorAdapter;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
@@ -153,15 +151,13 @@ public class BinderTests {
 	}
 
 	@Test
-	public void bindToValueWithMissingPlaceholdersShouldThrowException() {
+	public void bindToValueWithMissingPlaceholderShouldResolveToValueWithPlaceholder() {
 		StandardEnvironment environment = new StandardEnvironment();
 		this.sources.add(new MockConfigurationPropertySource("foo", "${bar}"));
 		this.binder = new Binder(this.sources,
 				new PropertySourcesPlaceholdersResolver(environment));
-		this.thrown.expect(BindException.class);
-		this.thrown.expectCause(ThrowableMessageMatcher.hasMessage(containsString(
-				"Could not resolve placeholder 'bar' in value \"${bar}\"")));
-		this.binder.bind("foo", Bindable.of(Integer.class));
+		BindResult<String> result = this.binder.bind("foo", Bindable.of(String.class));
+		assertThat(result.get()).isEqualTo("${bar}");
 	}
 
 	@Test

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/bind/PropertySourcesPlaceholdersResolverTests.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -67,12 +67,10 @@ public class PropertySourcesPlaceholdersResolverTests {
 	}
 
 	@Test
-	public void resolveIfPlaceholderAbsentAndNoDefaultShouldThrowException() {
+	public void resolveIfPlaceholderAbsentAndNoDefaultUsesPlaceholder() {
 		this.resolver = new PropertySourcesPlaceholdersResolver((PropertySources) null);
-		this.thrown.expect(IllegalArgumentException.class);
-		this.thrown
-				.expectMessage("Could not resolve placeholder 'FOO' in value \"${FOO}\"");
-		this.resolver.resolvePlaceholders("${FOO}");
+		Object resolved = this.resolver.resolvePlaceholders("${FOO}");
+		assertThat(resolved).isEqualTo("${FOO}");
 	}
 
 	@Test