Add 3.1 migration section to reference docs

Address error raised when using component-scan against the unqualified
"org" base package.

Issue: SPR-9843
Merge branch 'SPR-9843' into 3.2.x

* SPR-9843:
  Add 3.1 migration section to reference docs
Fix issue in BufferedImageHttpMessageConverter

This change ensures BufferedImageHttpMessageConverter writes when
"*/*" is requested.

Issue: SPR-7763
Add decoding matrix variable values

Issue: SPR-10140
Update mvc chapter on annotations and proxies

Issue: SPR-10132
Add support for HTTP OPTIONS in Spring MVC Test

Issue: SPR-10093
Remove closing tag in form tag's extra hidden fields

The fields returned from a RequestDataValueProcessor are now
written as hidden fields without a closing tag.

Before:
<input type="hidden" name="xxx" value="yyy"></input>

After:
<input type="hidden" name="xxx" value="yyy" />

Issue: SPR-10103
Merge pull request #205 from acogoluegnes/SPR-10093

* tmp:
  Add support for HTTP OPTIONS in Spring MVC Test
Update license header for recently modified files

Issue: SPR-7763, SPR-10140, SPR-10132, SPR-10093, SPR-10103
Update info on RestTemplate related to 401 status code

Issue: SPR-9367
Add option to always append 'must-revalidate'

Issue: SPR-9248
Add option to not trim path segments in AntPathMatch

Issue: SPR-8687
Add protected method to AbstractHandlerMethodMapping