Removes unnecessary check for nullness before call to g_free()
Maintains a reference to self while async writing request headers

in EvdHttpConnection.
Improves code readability in add() and remove() of EvdIoStreamGroup
Fixes add_internal() and remove_internal() of EvdIoStreamGroup

to return the actual result of EvdIoStream::set_group(), instead of always TRUE.
Fixes connection references in EvdWebsocketClient
Tracks close event of pooled connections in EvdConnectionPool

and adds other code improvements.