the default charset incorrect when the content-type is application/json

[slggamerTrue]

Environment details

1. HTTP Requests with the "content-type: applications/json" response
2. OS type and version: All
3. Java version: All
4. google-http-client version(s): All

Steps to reproduce

1. A Server response "content-type: application/json" with some special characters like Chinese.
2. From the https://tools.ietf.org/html/rfc4627, 3.Encoding "JSON text SHALL be encoded in Unicode. The default encoding is UTF-8."
3. But in google-http-java-client/google-http-client/src/main/java/com/google/api/client/http/HttpResponse.java, it use the charset Charsets.ISO_8859_1.

Thanks!