INT-2972 FixedMethodFilter - Return Mutable List

Compatibility with SF 3.2.2

Previously the filter returned a Collections.singletonList() when
a match is found.

Spring 3.2.2 attempts to sort the results of MethodFilter.filter()
which fails because the list is immutable.

Continue to return Collections.emptyList() with no match; this
does not cause the sort to fail.

Several tests in MethodInvokingMessageProcessorTests reproduce this
problem with Spring 3.2.2.

Add a test to confirm the Collections.emptyList() does not cause
any problems.
Merge pull request #792 from garyrussell/INT-2972

* INT-2972: FixedMethodFilter - Return Mutable List