Additional specs for img_prefix attribute
Additional specs for img_prefix attribute

These were previously tested in the context of img tag generation (see
img_spec.rb). Now we test the same thing, but in isolation using
attribute accessors.

Signed-off-by: Wincent Colaiuta <c76ac2e9cd86441713c7dfae92c451f3e6d17fdc@wincent.com>

wincent/wikitext,barnaclebarnes/wikitext,wincent/wikitext,barnaclebarnes/wikitext,geni/wikitext,geni/wikitext,barnaclebarnes/wikitext,geni/wikitext