/headers changing header capitalization.

[shadycuz]

This one cost me like 3 hours of debugging my application 😆

curl -X GET "https://httpbin.org/headers" -H "TestHeader: TestValue"

{
  "headers": {
    "Accept": "*/*", 
    "Host": "httpbin.org", 
    "Testheader": "TestValue", 
    "User-Agent": "curl/7.58.0", 
    "X-Amzn-Trace-Id": "Root=1-6053203b-2ca1ce7744e6a29f58f59021"
  }
}