only allow body to be consumed once, add some tests

classes/response.js

@@ -7,22 +7,32 @@ class Response {
     this.cacheFilePath = cacheFilePath;
     this.headers = new Headers(raw.headers);
     this.fromCache = fromCache;
+    this.consumed = false;
 
     if (this.bodyBuffer.type === 'Buffer') {
       this.bodyBuffer = Buffer.from(this.bodyBuffer);
     }
   }
 
+  consumeBody() {
+    if (this.consumed) {
+      throw new Error('Error: body used already');
+    }
+
+    this.consumed = true;
+    return this.bodyBuffer;
+  }
+
   text() {
-    return this.bodyBuffer.toString();
+    return this.consumeBody().toString();
   }
 
   json() {
-    return JSON.parse(this.bodyBuffer.toString());
+    return JSON.parse(this.consumeBody().toString());
   }
 
   buffer() {
-    return this.bodyBuffer;
+    return this.consumeBody();
   }
 
   async ejectFromCache() {