Fix a Clippy warning (Clippy 0.0.212 / Rust 1.38.0)

[tatsuya6502]

Clippy in the latest Rust stable emits the following warning:

error: You checked before that `unwrap()` cannot fail. Instead of checking and unwrapping, it's better to use `if let` or `match`.
  --> rustler/src/resource.rs:90:18
   |
88 |     if res.is_some() {
   |        ------------- the check is happening here
89 |         Some(ResourceType {
90 |             res: res.unwrap(),
   |                  ^^^^^^^^^^^^
   |
   = note: `-D clippy::unnecessary-unwrap` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_unwrap

This pull request addresses the warning by changing the codes as suggested by Clippy.

Version Info

Rust 1.38.0 stable

$ rustc -V
rustc 1.38.0 (625451e37 2019-09-23)

$ cargo clippy -V
clippy 0.0.212 (3aea860 2019-09-03)

[filmor]

There's an equivalent change already included in my pending PR #239 :)

[scrogson]

Thank you!

@filmor I'm merging this because CI is now failing because of this. Please rebase your changes on master.