`validate` does not handle UTF-8 byte order marker correctly

`frictionless.validate` treats "UTF-8 with BOM" as simply being UTF-8, which is not always correct; sometimes it is closer to ISO-8859-1 encoding.

If a non-UTF8 character appears in a file _after_ the first 10000 characters (ie the default sample size), then this will result in `validate` failing with an encoding error.
