Fix from previous PR. Pass file contents to Buffer.

This is broken in current node version.  'contents' is not a buffer, however 'file.contents' is a buffer.  Buffer.concat needs Buffers.
Re-wrote to support both streams and buffers.  This PR (https://github.com/tracker1/gulp-header/pull/3) broke gulp-header for buffer based uses.  We are an Angular shop and use the angular template gulp plugin (https://www.npmjs.org/package/gulp-angular-templatecache) which in turn depends on gulp-header.  The above PR broke the usage from templatecache.  Following Gulp's instructions for building plugins the following should support both stream and buffer use cases.
https://github.com/gulpjs/gulp/blob/master/docs/writing-a-plugin/README.md