INT-2832/2833 Aggregator Fix and Documentation

Expire empty groups.

Due to indentation changes, the code changes look more extensive
than they are. In effect the if (group.size() > 0) test is moved
to a narrower scope and the remove(group) is now performed if the
group is empty.

Document expire-groups-upon-completion.

INT-2832 Doc Polishing

PR Review + punctuation.

INT-2833 Add Delay For Expiring Empty Groups

minimumTimeoutForEmptyGroups
Merge pull request #683 from garyrussell/INT-2832-2833

* INT-2832-2833:
  INT-2832/2833 Aggregator Fix and Documentation
INT-2833 Fix Test Race Condition

Empty message group expiry used a < test instead of <=. This caused
the group in the test to be not deleted when it was expected to be.
INT-2843 - Document that with 2.2 Java serialization over HTTP is not enabled by default
INT-2842 - Doc: Re-order 'What's New in 2.2' section

For reference see:

* https://jira.springsource.org/browse/INT-2842
* https://jira.springsource.org/browse/INT-2843
Merge pull request #689 from ghillert/INT-2843

* INT-2843: Doc: with 2.2 Java serialization over HTTP not enabled by default

* INT-2842: Doc: Re-order 'What's New in 2.2' section
[artifactory-release] Release version 2.2.0.RELEASE
[artifactory-release] Next development version
INT-2844 Fix Http Test If Building With Spring 3.0

In Spring 3.1, DefaultResponseErrorHandler.getResponseBody() is
tolerant of a null InputStream on the response. With Spring 3.0
a null response causes an assertion exception.

The mock in the tests was not set up to return an InputStream
and so the test failed when building with Spring 3.0.

Configure the mock to return an empty InputStream.

Also added a text response to response.getStatusText().

Tested with Spring 3.0 and 3.1.
INT-2846: ImapMailReceiverTests: fix OS newLine

* Add `trim()` to the actual value
* Remove '\n' from expected value
* Remove redundant Mocks

JIRA: https://jira.springsource.org/browse/INT-2846
Merge pull request #692 from artembilan/INT-2846
INT-2845: Gradle: Check XSD Versions in Tests

In many cases IDEs import into test configs Namespace-resources
with hardcoded versions of Spring & Spring Integration XSDs.

This may produce build issues with different Spring versions.

It also causes an issue during the transition to a new Spring Integration version (e.g. 3.0).

* Add Gradle task 'checkTestConfigs' to check test configs for hardcoded resources' versions.
* Add to 'test' task dependency on new task 'checkTestConfigs'
* Fix hadrcoded versions in the test configs.

JIRA: https://jira.springsource.org/browse/INT-2845
Merge pull request #691 from artembilan/INT-2845
INT-2859 Fix Python OOM Error

INT-2859 - Remove Unnecessary Guard Condition
Merge pull request #694 from dturanski/INT-2859