Fix clippy field_reassign_with_default in generated code

[jan-auer]

In the next stable release, clippy will ship the field_reassign_with_default lint by default. This lint is also triggered in code generated by #[derive(JsonSchema)] when building schemars::schema::Metadata. This can be avoided by constructing the type in a single expression.

Instead of generating code like this:

let mut metadata = Metadata::default();
metadata.title = Some("title".to_owned());
metadata

This PR will generate code like this:

Metadata {
	title: Some("title".to_owned()),
	..Default::default()
}

Fixes #69

[jan-auer]

This lint is now enabled in clippy::all on Rust stable 1.49.0. @GREsau could you please take a look?

I've also briefly tried to find out why clippy is warning in generated code in the first place, but haven't been able to figure out how to suppress all checks.

[GREsau]

Thanks for the PR @jan-auer, sorry for the delay in merging!

[GREsau]

This is now in the latest version on crates.io - https://crates.io/crates/schemars/0.8.1

[webmaster128]

This is now in the latest version on crates.io - https://crates.io/crates/schemars/0.8.1

Thank you! This helps for clippy versions < 1.51.0 in which the problem will be fixed as well.