relax dependency on http_client mixin

bump to 5.2.3

Fix high CPU on retries in json_batch mode and enable compression (#89)

set default_codec doc attribute

add a bit more documentation around the json_batch format

Add empty event array check to multi_receive

Adds a check to return if #multi_receive is called with an empty array.
If this happens, the plugin will hang indefinitely on a #pop call that
will never return.

Fixes #80

Fixes #81

[skip ci] update gemspec summary

Bumping to 5.1 with changelog update

Fixes #73

bump to 4.4 for http_compression

Fixes #74

bump version (#71)