Fix duplicate bytes

[avoltz]

Fix #24

The logic which checks if content exceeds the block max has a bug where the last byte is sent again.

In a simpler form the bug is just:
x = 'test'
x[0..2] is 'tes'
x[2..] is 'st'

I think this never caused an issue with max len buffers because the constant subtracts 1, so there's room in the request to send that extra byte.

I applied this fix and ran the test described in #24 and the blob contents match the text file.

I also added some tests for this area.

[elsesiy]

Good catch @avoltz! @logachev can we get this merged and a new release cut?