INT-2958: XSD: move 'charset' from common attrs

`charset` for `<object-to-string-transformer>` was placed in the common `inputOutputChannelGroup` attributeGroup.
`charset` wasn't available  within `<chain>`.

* Move 'charset' from `inputOutputChannelGroup` to the new `specialized-transformer-charset-aware-type` complexType
* Add test for 'charset' within `<chain>`

JIRA: https://jira.springsource.org/browse/INT-2958
Merge pull request #764 from artembilan/INT-2958