Retrofit 2.1.0 发布 Android 的 REST 客户端

Retrofit 2.1.0 发布了，retrofit 是一个类型安全的 REST 客户端，用于 Android 平台。

改进日志如下：

New: @HeaderMap annotation and support for supplying an arbitrary number of headers to an endpoint.
New: @JsonAdapter annotations on the @Body parameter and on the method will be propagated to Moshi for creating the request and response adapters, respectively.
Fix: Honor the Content-Type encoding of XML responses when deserializing response bodies.
Fix: Remove the stacktrace from fake network exceptions created from retrofit-mock’sNetworkBehavior. They had the potential to be misleading and look like a library issue.
Fix: Eagerly catch malformed Content-Type headers supplied via @Header or @Headers.