Add 'Content-Type' header in IntoResponse

[dav-wolff]

Adds the appropriate 'Content-Type' header with value 'application/postcard' when generating a response in IntoResponse.

This makes sense as this crate already expects requests to include the Content-Type header, so it should also include it in its responses. This is also consistent with what axum's Json type does.
The documentation previously stated that the 'application/json' Content-Type was added to the response which was not the case. This is now updated.

I don't know if this crate is still being maintained, but it would be great to see this change on crates.io :)

[joseburgosguntin]

@dav-wolff thanks for the help :), fixed the test so we don't need the extra dependencies
note: it's available on crates.io with v0.2.0