[B] Update /say to vanilla behavior. Fixes BUKKIT-4224

**The issue**
Bukkit's implementation of the `/say` command doesn't match its vanilla counterpart. The following screenshots show the differences when running the command from in-game, then from a command block, then from the console and finally when you run the command with no arguments.

Vanilla:
![Vanilla](http://puu.sh/3u5zi.png)

Bukkit:
![Bukkit](http://puu.sh/3u5Jn.png)

**PR Rundown**
This PR makes Bukkit's implementation of the `/say` command look exactly the same as the vanilla one. I would show you a screenshot but there really is no point :wink: 

**Justification for this PR**
To quote from CONTRIBUTING.md:
> One of the goals of the Bukkit project is to be an extended Minecraft vanilla server
This PR makes Bukkit one step closer to being an extended Minecraft vanilla server!

**Related Links**
Jira: [BUKKIT-4224](https://bukkit.atlassian.net/browse/BUKKIT-4224)