Be explicit about how we determine content-length
Be explicit about how we determine content-length

In the acceptance test, we need to use a different method of working out
the size of the image for SVGs than other formats. Rather than falling
back if the `getResponseHeader` method fails, we are now explicit about
which method we use for each file type.

andybluntish/ember-cli-imagemin,andybluntish/ember-cli-imagemin