Only define arrayBuffer() if Blob is also supported

It turns out that Android 4.0 implements ArrayBuffer but no Blob.

This restores the behavior that v1.0 had, but which regressed in v1.1.0.

fetch.js

@@ -251,6 +251,14 @@
           return Promise.resolve(new Blob([this._bodyText]))
         }
       }
+
+      this.arrayBuffer = function() {
+        if (this._bodyArrayBuffer) {
+          return consumed(this) || Promise.resolve(this._bodyArrayBuffer)
+        } else {
+          return this.blob().then(readBlobAsArrayBuffer)
+        }
+      }
     }
 
     this.text = function() {
@@ -270,16 +278,6 @@
       }
     }
 
-    if (support.arrayBuffer) {
-      this.arrayBuffer = function() {
-        if (this._bodyArrayBuffer) {
-          return consumed(this) || Promise.resolve(this._bodyArrayBuffer)
-        } else {
-          return this.blob().then(readBlobAsArrayBuffer)
-        }
-      }
-    }
-
     if (support.formData) {
       this.formData = function() {
         return this.text().then(decode)