modify EncoderFrame return type.
commit b99a307ab94183c32a293ad5fda8b0e3323546a0
Author: ruil2 <ruil2@cisco.com>
Date:   Wed Mar 12 13:34:27 2014 +0800

    fix typo
modify unit test for return type modification
fix typo
Merge pull request #513 from ruil2/encoder_interface

Encoder interface