Accept link attributes without quote such as rel=next

The scanner was already setup to match the pattern, but there's an unnecessary gsub call in case of the match and caused runtime error such as:

```
> LinkHeader.parse('</path>; rel=next')
NoMethodError: undefined method `gsub' for nil:NilClass
       from {...}lib/ruby/gems/2.0.0/gems/link_header-0.0.7/lib/link_header.rb:94:in `parse'
```

This patch moves the gsub to only run in case of quoted match.