Fixed case of content-type header in nginx config

http* urls pointing to metadata files failed due to vagrant doing
an case-sensitive check for the content-type header to be application/json
to identify if the url points to a metadata file.
See: https://github.com/mitchellh/vagrant/blob/v1.6.3/lib/vagrant/action/builtin/box_add.rb#L488
Merge pull request #1 from tbal/master

Fixed case of content-type header in nginx config. Thanks @tbal