用AFNetworking的时候，报警告："Request failed: unacceptable content-type: text/plain"。百度出来说AFNetworking不支持text/plain，这个格式。
解决办法1.：在头文件"AFURLResponseSerialization.m"的如下地方添加text/plain就行了。问题解决。
self.acceptableContentTypes = [NSSet setWithObjects:@"application/json", @"text/plain",@"text/json", @"text/javascript", nil];
解决办法2.添加一句代码：
AFHTTPSessionManager *mgr = [AFHTTPSessionManager manager];
mgr.responseSerializer.acceptableContentTypes = [NSSet setWithObjects:@"text/plain",@"application/json", @"text/json", @"text/javascript",nil];
