Update src/stomp-specification-1.2.md

DISCONNECT may have an optional header `receipt`.
Merge pull request #60 from zhongl/patch-1

Update src/stomp-specification-1.2.md
cosmetic
fix typo
add RabbitMQ to 1.1 and 1.2 server list
The RabbitMQ server supports STOMP 1.1 as of version 2.3.0 and 1.2 as
of version 3.0.0.
Merge pull request #61 from rade/master

add RabbitMQ to 1.1 and 1.2 server list
added stompest to STOMP 1.2 clients (new section)
Merge pull request #63 from nikipore/master

added stompest to STOMP 1.2 clients
Fix markup.
rewritten from scratch, merging the version specific sections into comprehensive tables