INT-3067 Xslt method=text StringResult coercion

Previously there was a need to add `result-type="StringResult"`
to <int-xml:xslt-transformer> for xslt output `method='text'`
in case payload was non-standard type (e.g. `File`), if we wanted to get
result as text.

* Add coercion to `StringResult` code, if `Transformer`'s property `method` is 'text' and there is no explicit `resultFactory` provided.
* Add documentation to reference manual

JIRA: https://jira.springsource.org/browse/INT-3067