This change adds the ability to properly parse messages where the
content type is "multipart/alternative" into multiple Part objects.
Previously, the "multipart/alternative" content-type header would be
ignored, leaving alternative parts being rolled into one larger Part
object.

Signed-off-by: Matthew Setter <[email protected]>

This is a small change to correct some minor points in the related
documentation.

Signed-off-by: Matthew Setter <[email protected]>

I wasn't aware at the time that I created the previous commit with the
constant, of the existence of a constant with the same value. So this
commit removes my duplicate constant, replacing it with the existing
one.

Signed-off-by: Matthew Setter <[email protected]>

Co-authored-by: Matthew Weier O'Phinney <[email protected]>
Signed-off-by: Matthew Setter <[email protected]>

As per @MWOP's suggestion, this change properly typehints the protected
$parts property in Part to return an array of Part objects.

Signed-off-by: Matthew Setter <[email protected]>

Signed-off-by: Matthew Setter <[email protected]>

This change adds a series of assertions for attachments on
multipart/alternative messages, to help verify that they contain what
they're expected to contain. I felt that it was helpful to have
assertions for attachments (or non text/HTML content) as well, so that
more of the code could be properly tested.

Signed-off-by: Matthew Setter <[email protected]>

These were originally used because it was a fairly generic email that I
had at the time, while writing the tests. However, as per @MWOP's
suggestion, it should be more generic (and not preference a given,
commercial, email provider).

Signed-off-by: Matthew Setter <[email protected]>

This change simplistically sets a Part's filename property if the
content disposition header is set, it's an attachment, and the filename
property is set. I've deliberately avoided any considerations about
sanitising the filename, deciding, at this stage, to leave that to the
calling code up the line.

Signed-off-by: Matthew Setter <[email protected]>

Previously, the code only set a filename for a message part if the part
contained a content-disposition header where the disposition type was
set to "attachment" and the filename parameter was set. After re-reading
https://www.w3.org/Protocols/HTTP/Issues/content-disposition.txt, I see
that this was wrong, as the content-disposition header can have the
filename parameter set if the type is attachment or inline.

This commit corrects that earlier mistake.

Signed-off-by: Matthew Setter <[email protected]>