Reduce String creation in BeanPropertyRowMapper

Prior to this commit the BeanPropertyRowMapper used
String.substring and String.toLowerCase to parse the
field names. This would generate more String than needed.

Instead one could iterate over the internal char[] of the
String and use the Character methods instead. This reduces
the String creation.

Closes gh-25301

spring-jdbc/src/main/java/org/springframework/jdbc/core/BeanPropertyRowMapper.java

@@ -247,16 +247,15 @@ protected String underscoreName(String name) {
 		if (!StringUtils.hasLength(name)) {
 			return "";
 		}
+
 		StringBuilder result = new StringBuilder();
-		result.append(lowerCaseName(name.substring(0, 1)));
-		for (int i = 1; i < name.length(); i++) {
-			String s = name.substring(i, i + 1);
-			String slc = lowerCaseName(s);
-			if (!s.equals(slc)) {
-				result.append("_").append(slc);
+		for (int i = 0; i < name.length(); i++) {
+			char s = name.charAt(i);
+			if (Character.isUpperCase(s)) {
+				result.append('_').append(Character.toLowerCase(s));
 			}
 			else {
-				result.append(s);
+				result.append(Character.toLowerCase(s));
 			}
 		}
 		return result.toString();