improve p18 test cases

add muxed accounts, memo and grace period support for SEP-10

prepare for release 1.2.2

add null-safety eventsource code

update http & eventsource

SEP-10: add client attribution support

Update README.md

change representation of "..._muxed_id" field values to string - hori…

…zon 2.5.1

prepare for release 1.1.6

prepare for release 1.1.5