Buffer bugfix

Hello,
The latest version has bug in gearman_connection module.
#state.buffer is never overwritten with LeftOverData data. This error always leads to crash when We work with large workload data.
Here is the simple fix of this bug. https://github.com/orsis/erlang-gearman/commit/267938e5d261f4176a970382eabee5e28c8c3ac2 . And test which reproduces crach.

But I fork from 'rebar_migrate' branch. So, please, check this pull requet after We coordinate and merge 'Rebar migration' Pull request.

Thank You