Freeze all elements in HTML::Pipeline::SanitizationFilter
Freeze all elements in HTML::Pipeline::SanitizationFilter

We were recently burned by a regression that caused an exponential
number of transformers to be added to the pipeline because we didn't do
a deep copy of the whitelist. If these constants had been frozen, we
would have detected the error earlier.

jch/html-pipeline,gjtorikian/html-pipeline,jch/html-pipeline,gjtorikian/html-pipeline