MAINT charset is deprecated in favor of encoding

closes #2107
TST added tests for encoding/charset deprecation

Checking the old interface still works.
DOC better deprecation warning messages.

The charset parameter has been replaced by encoding and the charset_error by
decode_error.
TST catch deprecation warning in feature_extraction.text
Merge branch 'pr/2246'

MAINT charset is deprecated in favor of encoding