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

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.

Locate the MboxoReader implementation and the Python mailbox code that calls it. Reproduce a read with a size between 1 and 6, then determine the expected bounded-read behavior and add or update the relevant test so a read never returns more data than requested.

Written by the indexing model from the issue text.