create() is ambiguous (what it creates ?).

add several comments on what new() and _build_message() do.

replace data_type() with get_data_type().

define header_data_type() method to retrieve the data type of the
whole mail message.

we check preamble exists or not more rigorously to avoid to misread
broken(?) mulitpart data.

align types to be lowercase

"broken" implies the folloing type message:
* It says Content-Type: multipart but has no multipart block within it.
* multipart without close-delimiter

our analyzer loop does
do { alloc a new part + delimiter
} while ( not reach the end and non-zero data part ).

modify _get_next_pos() which returns (maxlen, maxlen) if broken
condition is satisfied. It means the next part has zero data, so
the next "alloc a new part" process is ignored in analyzer loop.

fix "text/rfc822-headers" type handling since
duplicated "text/rfc822-headers" parts may appear.
we check the duplication always.