Fix Clippy nags

[oherrala]

Fix Clippy nags. There should be no functional change.

One warning left unfixed since it would change the API:

warning: this argument is passed by value, but not consumed in the function body
  --> src/format.rs:95:51
   |
95 |   pub fn format_5424_structured_data(&self, data: StructuredData) -> String {
   |                                                   ^^^^^^^^^^^^^^ help: consider taking a reference instead: `&StructuredData`
   |
   = note: #[warn(needless_pass_by_value)] on by default
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.192/index.html#needless_pass_by_value

[Geal]

Thanks!