Fix #3914 - Inconsistent unicode names

This fixes https://github.com/rapid7/metasploit-framework/issues/3914

A few of the unicode names aren't defined correctly. For example: In the datastore option HTML::unicode, there is ```utf-32le```. However, the ```send_response_html``` method is actually checking ```utf32-le```, so if you choose that, you will trigger ```Exploit failed: The following options failed to validate: HTML::unicode```

## To verify that all HTML::unicode options work:

- [x] You can try this test module and place it in a browser directory: https://gist.github.com/wchen-r7/13d2c259acabb0c3d453
- [x] Run ```msfconsole```, load that module
- [x] Do ```show evasion```, make sure you see the ```HTML::unicode``` option
- [x] As long as you're using one of those acceptable values, you should NEVER see: ```Exploit failed: The following options failed to validate: HTML::unicode```