add header curl code example

Example 1: curl pass a header

curl -H "Accept-Charset: utf-8" -H "Content-Type: application/x-www-form-urlencoded" http://www.some-domain.com

Example 2: curl host header

curl --header "Host: example.com" http://127.0.0.1/

Example 3: curl headers

# -i to include response headers in the output.
# -H to specify a header
# -X to specify the request method
curl -i -H "Accept: application/json" -H "Content-Type: application/json" -X GET http://hostname/resource