fix(header): exporting test_header! macro
fix(header): exporting test_header! macro

That way, third-parties can keep using the `header!` macro, which
now requires the `test_header!` macro as well.

Considering the `header!` macro is exported and part of the public
API, it was not desired to prevent it's usage in any way.

nickgonzales/hyper,jmmaranan/hyper,fengsp/hyper,achanda/hyper,Keats/hyper,hyperium/hyper,giftcards/hyper,yazaddaruvala/hyper,yangxingpping/hyper,tshepang/hyper,golddranks/hyper,hannobraun/hyper,clatour/hyper,hannobraun/hyper,ianks/hyper,bonifaido/hyper,mlalic/hyper,mlalic/hyper,hyperium/hyper,tempbottle/hyper,nickgonzales/hyper,fhahn/hyper,rschulman/hyper,reem/hyper,lame-nickname/hyper,Twey/hyper,nickgonzales/hyper,nstoddard/hyper,fuchsnj/hyper,Twey/hyper,tshepang/hyper,mre/hyper,steveklabnik/hyper,yazaddaruvala/hyper,Keats/hyper,nstoddard/hyper,mneumann/hyper,Keats/hyper,pyfisch/hyper,golddranks/hyper,erickt/hyper,tshepang/hyper,bheesham/hyper,peterhj/hyper,mfeckie/hyper,markuskobler/hyper,markuskobler/hyper,Twey/hyper,youprofit/hyper,TimNN/hyper,manuel-woelker/hyper,hannobraun/hyper,sfackler/hyper,hyperium/hyper,softprops/hyper,golddranks/hyper,markuskobler/hyper,mitsuhiko/hyper