copy-as-curl should include content-type

[clintongormley]

With elastic/elasticsearch#22691, the content-type header will become mandatory for HTTP requests with bodies. copy-as-curl should be updated to include that header.

/cc @nik9000 in case you pick it up before i do

[nik9000]

/cc @nik9000 in case you pick it up before i do

I think I might be able to grab it now if you like.

[clintongormley]

thanks @nik9000