do not overwrite Content-Type Header when uploading gzip

Currently the grunt-s3-task is overwriting an existing 'Content-Type'-Header. This is kind of wrong, because you when you have overwritten it, you will check for the charset with the mime-module.

that only works for simple-text files. if i would like to upload 'application/javascript'-files, your default charset will be null. So i'm not able to upload gzipped js-files with UTF-8 charset.

i fixed that and only use the mime.lookup if the content-type-header is missing.

even if that issue wouldnt have arrised, i think it is not good to overwrite options which are settable from outside