Fixes Session#when_channel_polled to properly account for 4 byte length
header, so that it waits for the complete packet to arrive instead of
processing an incomplete packet and loosing sync with server.

A new test method, gets_packet_in_two, enables testing of fragmentation
edge cases.  expect_file_transfer now takes a :fragment_len parameter,
allowing for DRY testing of common fragmentation cases like this one.