wrap name in quotes to be able to identify empty strings as well

fetch.js

@@ -50,7 +50,7 @@ function normalizeName(name) {
     name = String(name)
   }
   if (/[^a-z0-9\-#$%&'*+.^_`|~!]/i.test(name) || name === '') {
-    throw new TypeError('Invalid character in header field name: ' + name)
+    throw new TypeError('Invalid character in header field name: "' + name + '"')
   }
   return name.toLowerCase()
 }