Add missing require to headers.rb
Add missing require to headers.rb

The delegate library is not auto-required in Ruby 2.2.x, so we need to explicitly require it if we are using SimpleDelegator.

jnunemaker/httparty,jnunemaker/httparty,ffleming/httparty,ffleming/httparty,jnunemaker/httparty,ffleming/httparty