Add tests for JSON Schema files (`.json`)

Add tests to check if JSON Schema files are serve compressed,
without any of the `HTML` document related headers, aren't cached,
and their specified character encoding is `UTF-8`, all of this if
the `.json` file is also served with the recommended media type,
namely, `application/schema+json`.

Ref https://github.com/topojson/topojson-specification
    h5bp/server-configs-apache#34
Update `CHANGELOG.md`
v0.4.0