do not modify when not Object::toString()

[joanvr]

What's changed?

Added a MethodMatcher for Object::toString() instead of checking simple name, to avoid matching on other methods called toString (like Arrays.toString(values))

What's your motivation?

Fixes https://github.com/moderneinc/support-app/issues/21

Anyone you would like to review specifically?

@knutwannheden @timtebeek

Checklist

• I've added unit tests to cover both positive and negative cases
• I've added the license header to any new files through ./gradlew licenseFormat
• I've used the IntelliJ IDEA auto-formatter on affected files