Use a string rather than symbol when reading recipe from a hash.

Closes projecthydra/hydra-derivatives#28

This and a couple of other small bugs were being masked by the fact that #kdu_compress_recipe always falls back to a default recipe:

https://github.com/jpstroop/hydra-derivatives/blob/add_jp2_support/lib/hydra/derivatives/jpeg2k_image.rb#L84-L98

simply testing that the stream had content was not sufficient. Additional tests to check that we're getting the intended recipe are included.