MimeParser parses closing boundary as normal boundary

[romain-grecourt]

When the closing delimiter of the closing boundary is delivered in a different chunk, MimeParser detects it as a normal boundary causing a parsing exception No closing MIME boundary.

Here is a simple test case to demonstrate the issue:

    @Test
    public void testClosingBoundaryAcrossChunks3() {
        String boundary = "boundary";
        final byte[] chunk1 = ("--" + boundary + "\n"
                + "Content-Id: part1\n"
                + "\n"
                + "this-is-the-body-of-part1\n"
                + "--" + boundary).getBytes();
        final byte[] chunk2 = "--".getBytes();

        List<MimePart> parts = parse(boundary, List.of(chunk1, chunk2)).parts;
        assertThat(parts.size(), is(equalTo(1)));

        MimePart part1 = parts.get(0);
        assertThat(part1.headers.size(), is(equalTo(1)));
        assertThat(part1.headers.get("Content-Id"), hasItems("part1"));
        assertThat(part1.content, is(notNullValue()));
        assertThat(new String(part1.content), is(equalTo("this-is-the-body-of-part1")));
    }