Update Default Spring Dependency to 3.2.6

Also usable with Spring 4.0.0 and 3.1.4.
INT-3233 NIO, Windows 7 and Java 7/8

JIRA: https://jira.springsource.org/browse/INT-3233

With Windows 7 and Java 7/8 closing a server channel does
not close the underlying socket. Closing the Selector does
close the socket.

It is not clear why this is needed, given that a selector can
be used for multiple sockets.

However, we only use the server side selector for a single
server socket so there is no detriment in closing it.

Also close the selector on the client side (even though it is
used for multiple sockets, because we are stopping the factory
anyway and all sockets will be closed).

However, closing the selector opens us up to 'ClosedSelectorException's
in several places. Add catch blocks to deal with this exception, and only
log an error if the factory is active.

While debugging this issue, I found that a number of (older) tests
left threads running, sockets open etc.

INT-3233 Polishing - PR Comments
INT-3235 Make GatewayMethodMetadata Public

JIRA: https://jira.springsource.org/browse/INT-3235

Unable to programmatically wire header expressions or method-specific
properties into a GatewayProxyFactoryBean.

The GPFB has public methods that take arguments of
this type.

Make the class public instead of package-protected.

Add a test case.