Writing tests and fix for #645.

[MatejNedic]

:loudspeaker: Type of change

• [x] Bugfix
• [ ] New feature
• [ ] Enhancement
• [ ] Refactoring

:scroll: Description

I only managed to reproduce error when in Json payload there would be contentTypes attribute "Value" without charset=UTF-8 "contentType": { "Type": "String", "Value": "application/json;" }, To fix it old ISO-8859-1 was replaced with StandardCharsets.UTF_8.name() .

:bulb: Motivation and Context

Fixing #645 .

:green_heart: How did you test it?

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [ ] I updated reference documentation to reflect the change
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps

[eddumelendez]

Thanks Matej! You can run ./mvnw io.spring.javaformat:spring-javaformat-maven-plugin:0.0.25:apply to fix the format issue.