Rework parsing of raw response HTTP headers

Now splits by `\r\n` because that's the header line delimiter.
No tests because I can't figure out how to return a manually formatted
headers payload from Node.js HTTP server.

Fixes #422

fetch.js

@@ -311,16 +311,17 @@
     return form
   }
 
-  function headers(xhr) {
-    var head = new Headers()
-    var pairs = (xhr.getAllResponseHeaders() || '').trim().split('\n')
-    pairs.forEach(function(header) {
-      var split = header.trim().split(':')
-      var key = split.shift().trim()
-      var value = split.join(':').trim()
-      head.append(key, value)
+  function parseHeaders(rawHeaders) {
+    var headers = new Headers()
+    rawHeaders.split('\r\n').forEach(function(line) {
+      var parts = line.split(':')
+      var key = parts.shift().trim()
+      if (key) {
+        var value = parts.join(':').trim()
+        headers.append(key, value)
+      }
     })
-    return head
+    return headers
   }
 
   Body.call(Request.prototype)
@@ -375,26 +376,13 @@
       var request = new Request(input, init)
       var xhr = new XMLHttpRequest()
 
-      function responseURL() {
-        if ('responseURL' in xhr) {
-          return xhr.responseURL
-        }
-
-        // Avoid security warnings on getResponseHeader when not allowed by CORS
-        if (/^X-Request-URL:/mi.test(xhr.getAllResponseHeaders())) {
-          return xhr.getResponseHeader('X-Request-URL')
-        }
-
-        return
-      }
-
       xhr.onload = function() {
         var options = {
           status: xhr.status,
           statusText: xhr.statusText,
-          headers: headers(xhr),
-          url: responseURL()
+          headers: parseHeaders(xhr.getAllResponseHeaders() || '')
         }
+        options.url = 'responseURL' in xhr ? xhr.responseURL : options.headers.get('X-Request-URL')
         var body = 'response' in xhr ? xhr.response : xhr.responseText
         resolve(new Response(body, options))
       }