fix clippy warnings

[AnderEnder]

Example:

warning: called `.as_ref().map(String::as_str)` on an Option value. This can be done more directly by calling `self.email.as_deref()` instead
  --> src/extension/itunes/itunes_owner.rs:73:9
   |
73 |         self.email.as_ref().map(String::as_str)
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using as_deref instead: `self.email.as_deref()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#option_as_ref_deref

[codecov]

Codecov Report

Merging #93 into master will not change coverage.
The diff coverage is 97.77%.

@@           Coverage Diff           @@
##           master      #93   +/-   ##
=======================================
  Coverage   79.52%   79.52%           
=======================================
  Files          24       24           
  Lines        1724     1724           
=======================================
  Hits         1371     1371           
  Misses        353      353           

Impacted Files	Coverage Δ
src/channel.rs	79.28% <88.88%> (ø)
src/category.rs	91.66% <100.00%> (ø)
src/extension/itunes/itunes_category.rs	100.00% <100.00%> (ø)
src/extension/itunes/itunes_channel_extension.rs	82.66% <100.00%> (ø)
src/extension/itunes/itunes_item_extension.rs	85.50% <100.00%> (ø)
src/extension/itunes/itunes_owner.rs	100.00% <100.00%> (ø)
src/extension/mod.rs	71.42% <100.00%> (ø)
src/image.rs	82.00% <100.00%> (ø)
src/item.rs	81.10% <100.00%> (ø)
src/source.rs	91.66% <100.00%> (ø)

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