SpringBoot Detection of PeriodStyle.ISO8601 does not support lower-case input

Support lower-case input in PeriodStyle for ISO-8601 format used by java.time.Period.

This is a pull request similar to pull request #32223.

It will fix a bug similar to the one reported as #32218, found in Spring Boot version 2.7.3 with JDK 11.

The bug is demonstrated in the example application at https://github.com/valentine-dev/spring-boot-period-style-iso8601.

Comment From: snicoll

Thanks again @valentine-dev

Comment From: valentine-dev

@snicoll My pleasure.