if user has some custom subclass of Image pipeline and no setting for
this pipeline, he should get default settings defined for Image Pipeline.
Fixes#2198
some class attributes for ImagePipeline and FilesPipeline had DEFAULT prefix. These
attributes should be preserved as well, if users subclasses define values for
DEFAULT_<CLS_ATTRIBUTE_NAME> attribute this value should be preserved.
there was identical test for different setting keys. I unified it into
one unit test.
Fixes comments for tests, adds comments about intention of uppercase attrs.
Adds another test for user defined setting keys and uppercase attrs.
allow users to have class attributes on image pipelines. This assumes
that class attributes are useful if users want to have different pipeline
classes inhriting from ImagePipeline.
