Bring coverage up to 100%

.babelrc

@@ -13,7 +13,12 @@
         [ "es2015", { "loose": true } ]
       ],
       "plugins": [
-        "istanbul"
+        [ "istanbul", {
+          "exclude": [
+            "src/blob.js",
+            "test"
+          ]
+        } ]
       ]
     },
     "rollup": {

src/headers.js

@@ -245,6 +245,7 @@ function createHeadersIterator(target, kind) {
 
 const HeadersIteratorPrototype = Object.setPrototypeOf({
 	next() {
+		// istanbul ignore if
 		if (!this ||
 			Object.getPrototypeOf(this) !== HeadersIteratorPrototype) {
 			throw new TypeError('Value of `this` is not a HeadersIterator');