Merge branch 'jc/same-encoding' into maint

Various codepaths checked if two encoding names are the same using ad-hoc code and some of them ended up asking iconv() to convert between "utf8" and "UTF-8". The former is not a valid way to spell the encoding name, but often people use it by mistake, and we equated them in some but not all codepaths. Introduce a new helper function to make these codepaths consistent. * jc/same-encoding: reencode_string(): introduce and use same_encoding()