API: Clarify Access-Control-Request-Header handling
API: Clarify Access-Control-Request-Header handling

'headers' was a way too generic name, and the code structure implied
it was used in place of 'resp.headers' or 'request.headers', which is not the case.
Also add comment about the purpose of this code.

CapnBry/OctoPrint,beeverycreative/BEEweb,eliasbakken/OctoPrint,Jaesin/OctoPrint,senttech/OctoPrint,mcanes/OctoPrint,foosel/OctoPrint,dragondgold/OctoPrint,madhuni/AstroBox,leductan-nguyen/RaionPi,Jaesin/OctoPrint,javivi001/OctoPrint,CapnBry/OctoPrint,spapadim/OctoPrint,markwal/OctoPrint,shohei/Octoprint,nickverschoor/OctoPrint,Javierma/OctoPrint-TFG,CapnBry/OctoPrint,ymilord/OctoPrint-MrBeam,dansantee/OctoPrint,Salandora/OctoPrint,javivi001/OctoPrint,abinashk-inf/AstroBox,eddieparker/OctoPrint,ryanneufeld/OctoPrint,sstocker46/OctoPrint,Javierma/OctoPrint-TFG,abinashk-inf/AstroBox,dragondgold/OctoPrint,dragondgold/OctoPrint,alex1818/OctoPrint,masterhou/OctoPrint,foosel/OctoPrint,jneves/OctoPrint,leductan-nguyen/RaionPi,Mikk36/OctoPrint,Mikk36/OctoPrint,uuv/OctoPrint,shaggythesheep/OctoPrint,spapadim/OctoPrint,chriskoz/OctoPrint,mayoff/OctoPrint,dansantee/OctoPrint,beeverycreative/BEEweb,shohei/Octoprint,Javierma/OctoPrint-TFG,ErikDeBruijn/OctoPrint,Skeen/OctoPrint,abinashk-inf/AstroBox,markwal/OctoPrint,Voxel8/OctoPrint,ErikDeBruijn/OctoPrint,eddieparker/OctoPrint,chriskoz/OctoPrint,nicanor-romero/OctoPrint,MaxOLydian/OctoPrint,alex1818/OctoPrint,madhuni/AstroBox,EZ3-India/EZ-Remote,ymilord/OctoPrint-MrBeam,eddieparker/OctoPrint,senttech/OctoPrint,mcanes/OctoPrint,Voxel8/OctoPrint,CapnBry/OctoPrint,nickverschoor/OctoPrint,shohei/Octoprint,ymilord/OctoPrint-MrBeam,MoonshineSG/OctoPrint,mrbeam/OctoPrint,Jaesin/OctoPrint,JackGavin13/octoprint-test-not-finished,nicanor-romero/OctoPrint,ryanneufeld/OctoPrint,nickverschoor/OctoPrint,SeveQ/OctoPrint,ymilord/OctoPrint-MrBeam,MoonshineSG/OctoPrint,beeverycreative/BEEweb,ryanneufeld/OctoPrint,spapadim/OctoPrint,senttech/OctoPrint,Jaesin/OctoPrint,dansantee/OctoPrint,leductan-nguyen/RaionPi,AstroPrint/AstroBox,ryanneufeld/OctoPrint,JackGavin13/octoprint-test-not-finished,punkkeks/OctoPrint,punkkeks/OctoPrint,masterhou/OctoPrint,MoonshineSG/OctoPrint,MaxOLydian/OctoPrint,SeveQ/OctoPrint,JackGavin13/octoprint-test-not-finished,Salandora/OctoPrint,senttech/OctoPrint,MoonshineSG/OctoPrint,Voxel8/OctoPrint,mrbeam/OctoPrint,mcanes/OctoPrint,shaggythesheep/OctoPrint,EZ3-India/EZ-Remote,masterhou/OctoPrint,Javierma/OctoPrint-TFG,beeverycreative/BEEweb,eliasbakken/OctoPrint,shohei/Octoprint,sstocker46/OctoPrint,EZ3-India/EZ-Remote,Skeen/OctoPrint,punkkeks/OctoPrint,nicanor-romero/OctoPrint,nickverschoor/OctoPrint,jneves/OctoPrint,JackGavin13/octoprint-test-not-finished,ErikDeBruijn/OctoPrint,bicephale/OctoPrint,foosel/OctoPrint,markwal/OctoPrint,MaxOLydian/OctoPrint,bicephale/OctoPrint,uuv/OctoPrint,abinashk-inf/AstroBox,Salandora/OctoPrint,chriskoz/OctoPrint,AstroPrint/AstroBox,EZ3-India/EZ-Remote,ymilord/OctoPrint-MrBeam,jneves/OctoPrint,chriskoz/OctoPrint,Mikk36/OctoPrint,leductan-nguyen/RaionPi,SeveQ/OctoPrint,uuv/OctoPrint,foosel/OctoPrint,Skeen/OctoPrint,bicephale/OctoPrint,madhuni/AstroBox,bicephale/OctoPrint,mrbeam/OctoPrint,shaggythesheep/OctoPrint,Salandora/OctoPrint,sstocker46/OctoPrint,eliasbakken/OctoPrint,shohei/Octoprint,mayoff/OctoPrint,mayoff/OctoPrint,AstroPrint/AstroBox,alex1818/OctoPrint,madhuni/AstroBox,javivi001/OctoPrint