Unicode characters in response body does not encoded correctly

[wkh237]

When response data contains unicode characters response.json() and response.text() does not convert unicode character correctly. We should check if the response data is a valid UTF8 string before encode it to BASE64 string. When it's valid, use URL encoding before BASE64.

• Android
• IOS
• Test case

[wkh237]

close on 0.9.0 release