Make msg method of DerivativeContext work properly.

Previously, the msg method was not taking into account any
changes to the context, and was just using the original context.

For example, if you had a DerivativeContext object, where the
original context had a language of en, and then you did
$derivContext->setLanguage( 'fr' );, $derivContext->msg( 'foo' )
was still outputting stuff in english instead of french.

Change-Id: I4a87e0e2664e77bf79a80c873db384e0c3f607e7