Rename local variable headers to hdrs
Rename local variable headers to hdrs

Fixes the following warning when run under -w:
  aws-sigv4-1.0.2/lib/aws-sigv4/signer.rb:441:
    warning: shadowing outer local variable - headers

aws/aws-sdk-ruby,llooker/aws-sdk-ruby,aws/aws-sdk-ruby,aws/aws-sdk-ruby,llooker/aws-sdk-ruby,llooker/aws-sdk-ruby