deflater: rely on frozen_string_literal
deflater: rely on frozen_string_literal

This improves readability of our code and can allow us to
generate less garbage in Ruby 2.3+ for places where we didn't
already use constants.  We can also avoid the old constant
lookups (and associated inline cache overhead) this way.

gfvcastro/rack,gfvcastro/rack,gfvcastro/rack