Bug: MboxoReader can return more data than requested if 0 < size < 7

[sebbASF]

To simplify the MboxoReader code, it currently forces a read of less than 7 bytes to read 7 bytes.
The response can potentially return more bytes than requested.

The existing Python mailbox code which calls it either reads everything or uses a size of 8192, so this should not be a problem in practise, but it should be at least noted that the code is not completely generic.

[sebbASF]

Note: the simplification only really works for mboxo format mailboxes.
A different approach would be needed to support mboxrd format as this can have an arbitrary number of '>' chars before the From.