Import of data failed when sending special chars (umlaute)

When sending special chars (umlaute like öäü) during a data import the server returns 
com.mixpanel.mixpanelapi.MixpanelServerException: Server refused to accept messages, they may be malformed.

The problem is solved by using the method getBytes("utf-8") in the Base64Coder instead of getBytes().
 
getBytes()  encodes the json string with the default char set which is not necessarily utf-8.
see http://docs.oracle.com/javase/6/docs/api/java/lang/String.html#getBytes%28%29

TODO: check if there are side effects in other uses of the method encodeString