Improve text and blobs detection from protocol type and character set.

MySQL sends BLOB and TEXT types under the same column types in the
protocol, as per the following documentation:

  http://dev.mysql.com/doc/internals/en/com-query-response.html#column-type

As we can see in https://github.com/dimitri/pgloader/issues/106 it was
wrong to mix the special character set +mysql-cs-coll-binary+ used to
decide if we just received binary or text data with other _bin
collaction, used for something else entirely, as detailed in:

  http://dev.mysql.com/doc/refman/5.0/en/blob.html