Allow more kind of object in mock

[shingara]

With a mock you allow return a symbol or an Hash. But with our test we
can want return some Savon::SOAP::Response or HTTPI::Response save by
vcr for example.

This commit allow to return this kind of Object. You can do something
like :

resources = VCR.use_cassette('get_resources') do
  Savon.client(wsdl).request(:get_resouces)
end

savon.expects(:get_resources).returns(resources)