Add recognized HTTP headers

I use S3 for static website hosting and want to add X-Robots-Tag HTTP header to some files. I was not able to  do this with boto, as it the header turned into x-amz-meta-x-robots-tag; not what I desired. However with this simple change to boto I was able to achieve what I wanted. While I was at it I did the same change for Content-Language header as well.

I don't completely understand when the x-amz-meta tag is necessary so there may be some catch, but at first glance this seems to work perfectly. Also it seems that this fixes bug https://github.com/boto/boto/issues/2050, although I'm not completely sure why. Then again, you might want to fix that bug for any metadata, not only for Content-Encoding.