cased -> uncased in BERT GLUE example
similar to https://github.com/huggingface/transformers/issues/3183, the GLUE readme also have this issue, the MRPC example use `bert-base-cased` while at the same time `--do_lower_case`.
additionally, the `xlnet-large-cased` should not combine with `--do_lower_case` as `xlnet` model only has a cased version.