Add 'additionalContextAttributes.{name}={value}' instead of 'additional.context.attributes' 

I've change the property key for specifying the additional context attributes for support configuration property on mybatis-spring-boot . The `additional.context.attributes` can be use, however it become the deprecated property key since 2.1.0. If you are updating to 2.1+, please change to use the `additionalContextAttributes.{name}={value}` instead of it.