Work around JuiceSSH rendering bug

JuiceSSH apparently has a bug where ECH for one character (ESC [ 1 X) does
not actually erase the character, in its code that receives and interprets
Mosh state updates.  This was hidden before because Mosh <= 1.2.5 never sent
this sequence, it sent ESC [ X instead as an optimization.

Do the better optimization of sending spaces for short sequences of blanks
instead.