Overloading MunitSpy withAttributes(...) to accept Attibutes object as a parameter.

Overloading MunitSpy withAttributes(...) to accept Attibutes object as
a parameter  as well as a Map to be consistent with
MessageProcessorMocker withAttributes(…).
Tidy test method.
Merge pull request #66 from ryandcarter/munit-3.4.x

Overloading MunitSpy withAttributes(...) to accept org.mule.munit.common.mocking.Attribute object as a parameter to be consistent with MessageProcessorMocker withAttributes(…).