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ARROW-5129: [Rust] Column writer bug: check dictionary encoder when adding a new data page

This PR fixes the issue when we would encode all data into a single data page, unless we fall back to secondary encoder. Added test to verify the correctness of the patch.

Test is a bit awkward and clunky, but that is the only way I could come up to test the patch, since I need to check the written pages (I could not get "adding buffer to TestPageWriter" to work because of the box + lifetime issues).

Author: Ivan Sadikov <ivan.sadikov@gmail.com>

Closes #4152 from sadikovi/ARROW-5129 and squashes the following commits:

61d2741b <Ivan Sadikov> fix style
42eb4de8 <Ivan Sadikov> add test
be7f41a8 <Ivan Sadikov> check dict encoder for data pages