register ajax just as a plain mime type.

[jwoertink]

Purpose

Fixes #1320

Description

If you wanted to set your desired format to be ajax, but you're not sending JSON or XML, then we just set it to plain and return the raw text.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test