LightGBM

Support UTF-8 characters in feature name again (#2976)

* Support UTF-8 characters in feature name again

This commit reverts 0d59859c670b9de37bffa8a6e536497c88d9f25d.
Also see:
- https://github.com/microsoft/LightGBM/issues/2226
- https://github.com/microsoft/LightGBM/issues/2478
- https://github.com/microsoft/LightGBM/pull/2229

I reproduced the issue and as @kidotaka gave us a great survey in #2226,
I don't conclude that the cause is UTF-8, but "an empty string (character)".
Therefore, I revert "throw error when meet non ascii (#2229)" whose commit hash
is 0d59859c670b9de37bffa8a6e536497c88d9f25d, and add support feture names as UTF-8 again.

* add tests

* fix check-docs tests

* update

* fix tests

* update .travis.yml

* fix tests

* update test_r_package.sh

* update test_r_package.sh

* update test_r_package.sh

* add a test for R-package

* update test_r_package.sh

* update test_r_package.sh

* update test_r_package.sh

* fix test for R-package

* update test_r_package.sh

* update test_r_package.sh

* update test_r_package.sh

* update test_r_package.sh

* update

* updte

* update

* remove unneeded comments