revert: revert converting header names to lowercase

lib/plugin.js

@@ -1,15 +1,9 @@
 import KY from 'ky-universal'
 
 class HTTP {
-  constructor({ headers, ...defaults }, ky = KY) {
-    const _headers = {}
-    for (const name in headers) {
-      _headers[name.toLowerCase()] = headers[name]
-    }
-
+  constructor(defaults, ky = KY) {
     this._defaults = {
       hooks: {},
-      headers: _headers,
       ...defaults
     }
 
@@ -17,8 +11,6 @@ class HTTP {
   }
 
   setHeader(name, value) {
-    name = name.toLowerCase()
-
     if (!value) {
       delete this._defaults.headers[name];
     } else {