fix(hydrate): support headers serialization from string

lib/hydrate.js

@@ -7,17 +7,43 @@ value should have the following format
     status: '',
     statusText: ''
   },
-  headers: {
-    // map of HTTP response headers
-  }
+  headers: ''
 }
 */
+
+// borrow from superagent
+function trim (s) {
+  return ''.trim ? s.trim(s) : s.replace(/(^\s*|\s*$)/g, '')
+}
+
+// borrow from superagent
+function parseHeader (str) {
+  var lines = str.split(/\r?\n/)
+  var fields = {}
+  var index
+  var line
+  var field
+  var val
+
+  lines.pop() // trailing CRLF
+
+  for (let i = 0, len = lines.length; i < len; i += 1) {
+    line = lines[i]
+    index = line.indexOf(':')
+    field = line.slice(0, index).toLowerCase()
+    val = trim(line.slice(index + 1))
+    fields[field] = val
+  }
+
+  return fields
+}
+
 export default function hydrate (value) {
-  const headers = data.headers || {}
   const xhr = value.body || {}
+  const headers = parseHeader(value.headers || {})
 
   xhr.getAllResponseHeaders = function () {
-    return headers
+    return value.headers
   }
 
   xhr.getResponseHeader = function (header) {