HV-1140 Use ROOT locale in StringHelper#decapitalize

Before that, we could have locale specific behaviors when lowercasing.
It was especially true for the Turkish locale - which is often cited as
an example of this issue.

Author: Mert ÇALIŞKAN

engine/src/main/java/org/hibernate/validator/internal/util/StringHelper.java

@@ -7,6 +7,7 @@
 package org.hibernate.validator.internal.util;
 
 import java.util.Arrays;
+import java.util.Locale;
 
 /**
  * Helper class dealing with strings.
@@ -81,7 +82,7 @@ public static String decapitalize(String string) {
 			return string;
 		}
 		else {
-			return string.substring( 0, 1 ).toLowerCase() + string.substring( 1 );
+			return string.substring( 0, 1 ).toLowerCase( Locale.ROOT ) + string.substring( 1 );
 		}
 	}
 

engine/src/test/java/org/hibernate/validator/test/internal/util/StringHelperTest.java

@@ -7,7 +7,9 @@
 package org.hibernate.validator.test.internal.util;
 
 import java.util.Arrays;
+import java.util.Locale;
 
+import org.hibernate.validator.testutil.TestForIssue;
 import org.testng.annotations.Test;
 
 import org.hibernate.validator.internal.util.StringHelper;
@@ -87,6 +89,15 @@ public void decapitalizeShouldReturnDecapizalizedWord() {
 		assertEquals( StringHelper.decapitalize( "Giraffe" ), "giraffe" );
 	}
 
+	@Test
+	@TestForIssue(jiraKey = "HV-1140")
+	public void decapitalizeShouldReturnDecapizalizedWordOnTurkishLocale() {
+		Locale defaultLocale = Locale.getDefault();
+		Locale.setDefault( new Locale( "tr" , "TR" ) );
+		assertEquals( StringHelper.decapitalize( "IsIsolationLevelGuaranteed" ), "isIsolationLevelGuaranteed" );
+		Locale.setDefault( defaultLocale );
+	}
+
 	@Test
 	public void decapitalizeShouldReturnSameWordForDecapizalizedWord() {
 		assertEquals( StringHelper.decapitalize( "giraffe" ), "giraffe" );