add unit tests (export object to all types) (issue #7)

This version introduces multiple enhancements:

all export object tests

Details of additions/deletions below:
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Modified:   Test/ObjectTest.php
            -- Added --
                testExportObjectAsString export object as string (data separated by defined separator) data with data return callback
                testExportObjectAsSerializedString export object as serialized string with data return callback
                testExportObjectAsSerializedStringWithObject export object as serialized string (with object) with data return callback
                testExportObjectAsXml export object as xml with data return callback