fix(serde_conv): allow clippy::ptr_arg

[mkroening]

This is #320 again after the allow was removed in f3892d0.

This is the offending code that I have:

serde_with::serde_conv!(
    pub StringAsHtml,
    String,
    |string: &str| html_escape::encode_text(string).into_owned(),
    |html: String| -> Result<_, Infallible> {
        match html_escape::decode_html_entities(&html) {
            Cow::Owned(string) => Ok(string),
            Cow::Borrowed(_) => Ok(html),
        }
    }
);

This results in:

warning: writing `&String` instead of `&str` involves a new object where a slice will do
   --> serde_with/serde_with/src/serde_conv.rs:116:41
    |
107 |   macro_rules! serde_conv {
    |   ----------------------- in this expansion of `serde_with::serde_conv!`
...
116 |                   $vis fn serialize<S>(x: &$t, serializer: S) -> $crate::__private__::Result<S::Ok, S::Error>
    |                                           ^^^
    |
   ::: src/serde.rs:31:1
    |
31  | / serde_with::serde_conv!(
32  | |     pub StringAsHtml,
33  | |     String,
34  | |     |string: &str| html_escape::encode_text(string).into_owned(),
...   |
40  | |     }
41  | | );
    | |_- in this macro invocation
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
    = note: `#[warn(clippy::ptr_arg)]` on by default

I am not sure if there is a way to resolve the issue on my end.
What do you think?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.49%. Comparing base (f4145e3) to head (4a5a4ab).
Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #729   +/-   ##
=======================================
  Coverage   66.49%   66.49%           
=======================================
  Files          38       38           
  Lines        2468     2468           
=======================================
  Hits         1641     1641           
  Misses        827      827           

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[jonasbb]

Sorry that it regressed. I added a test in #731 such that it hopefully doesn't regress anymore.