Don't assume `upload.src` is a simple string.

Setting `upload.src` to an array of globs (as per the [grunt.file.expand](http://gruntjs.com/api/grunt.file#grunt.file.expand) docs) causes an "Object has no method 'replace'" error.

This fixes a simple assumption that upload.src will simply be a string. After the fix, some complex globbing patterns I wrote now work fine, which makes grunt-s3 much more powerful.