read mysql text column (utf8 charset) as string column

[Cabbagec]

Encountered with the same problem as #464 and #510 , where text type columns in mysql are interpreted as binary ones naively, which is technically ok, according to the mysql doc that the text types are basically just blob types, except for additional charset requirements.

But it's quite an inconvenient for users must do decoding themselves on the final result. So I made and tested this patch in our environment as an easy workaround. Note that we only use UTF8 charset for text fields in our databases, so this patch only works for UTF8 text fields. For other encodings like GBK, SHIFT_JIS, BIG5, etc., it won't work, additional decoding is still needed.

If anyone interested, I recommend checking out this crate encoding_rs, then doing decoding steps according to the mysql field charset info.

[wangxiaoying]

Thanks @Cabbagec for the PR and testing!