CSV containing SVG tags incorrectly detected as image/svg+xml 

Related issue: #107

CSV files with SVG tags are incorrectly detected as SVGs (`image/svg+xml`) 

```ruby
Marcel::MimeType.for Pathname.new("test.csv")
=> "image/svg+xml"
```

**Input:**
`test.csv`
```
title,width,height,body
Test,100,100,"<svg></svg>"
```