[patch] Support "accept-charset" attribute in HTML::Form

The patch is attached.

This attribute is rarely used in HTML but the interesting functionality is
encoding character bodies of generated requests into bytes according to the
specified encoding.

If you try to click() a form specified as <form method="POST"
enctype="multipart/form-data"> with some Unicode data in its fields you will
get this error:
HTTP::Message content must be bytes at blib/lib/HTTP/Request/Common.pm line
91

My patch fixes that by always encoding wide characters to the encoding
specified in "accept-charset" attribute or set via method. Defaults to
utf-8. Avoids unneeded encodings.

Also, caution taken not to break compatibility with older perls.

Docs and tests included.