When doing local oneboxes we sometimes want to allow
SVGs in the final preview HTML. The main case currently
is for the new cooked hashtags, which include an SVG
icon.
SVGs will be included in local oneboxes via `ExcerptParser` _only_
if they have the d-icon class, and if the caller for `post.excerpt`
specifies the `keep_svg: true` option.
It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
