Fix CSP copy boolean directives

[spdawson]

Summary

Copying a CSP instance fails if the instance contains Boolean directives; specifically, this affects the block_all_mixed_content, upgrade_insecure_requests and sandbox directives.

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[jeremy]

Use Hash#deep_dup? https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/object/deep_dup.rb

[jeremy]

Can do directives.deep_dup directly. At which point I'd just get rid of the method and use #deep_dup directly in #initialize_copy:

def initialize_copy(other)
  @directives = other.directives.deep_dup
end