fix: Headers only accepts array which have nested array of length 2

fixes https://github.com/JakeChampion/fetch/issues/1235

fetch.js

@@ -92,6 +92,9 @@ export function Headers(headers) {
     }, this)
   } else if (Array.isArray(headers)) {
     headers.forEach(function(header) {
+      if (header.length != 2) {
+        throw new TypeError('Headers constructor: expected name/value pair to be length 2, found' + header.length)
+      }
       this.append(header[0], header[1])
     }, this)
   } else if (headers) {

test/test.js

@@ -203,6 +203,16 @@ exercise.forEach(function(exerciseMode) {
 
         assert.equal(headers.get('Content-Type'), 'text/xml')
         assert.equal(headers.get('Breaking-Bad'), '<3')
+        assert.throws(function() {
+          new Headers([
+            ['Content-Type'],
+          ])
+        }, TypeError)
+        assert.throws(function() {
+          new Headers([
+            ['Content-Type', 'a', 'b'],
+          ])
+        }, TypeError)
       })
       test('headers are case insensitive', function() {
         var headers = new Headers({Accept: 'application/json'})