fix underscore casing to transform Name to name, and not _name

Author: kimchy

modules/elasticsearch/src/main/java/org/elasticsearch/common/Strings.java

@@ -1220,9 +1220,16 @@ public static String toUnderscoreCase(String value, StringBuilder sb) {
                         sb.append(value.charAt(j));
                     }
                     changed = true;
+                    if (i == 0) {
+                        sb.append(Character.toLowerCase(c));
+                    } else {
+                        sb.append('_');
+                        sb.append(Character.toLowerCase(c));
+                    }
+                } else {
+                    sb.append('_');
+                    sb.append(Character.toLowerCase(c));
                 }
-                sb.append('_');
-                sb.append(Character.toLowerCase(c));
             } else {
                 if (changed) {
                     sb.append(c);

modules/elasticsearch/src/test/java/org/elasticsearch/util/StringsTests.java

@@ -44,6 +44,8 @@ public class StringsTests {
         String testValue = "test_value";
         assertThat(toUnderscoreCase(testValue), equalTo(testValue));
         assertThat(toUnderscoreCase(testValue), sameInstance(testValue));
+
+        assertThat(toUnderscoreCase("Name"), equalTo("name"));
     }
 
 //    @Test public void testHasTextBlank() throws Exception {