Fix "ignore-case" behavior (#339)

Summary:
Currently, if `ignore-case` is set, the same line will be yielded twice - once as lower-cased version, once as original version, leading to lower than expected uncased scores.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/339

Differential Revision: D12890386

Pulled By: myleott

fbshipit-source-id: 0570e5f6e8f848f2c6439d615e70aca6df097eef