add assertions into test_getter
suppress interpreter warnings

Why
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This commit removes the following interpreter warning.

* warning: instance variable @{ivar} not initialized

Reproduction
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* ruby 1.9.3p392 (2013-02-22 revision 39386)
* `$VERBOSE = true` on test
fix reader methods

Fixes #8
Merge pull request #9 from kachick/fix-#8

Reader methods should return nil when accessing unknown attributes