fix: specifically use serde::Serialize in Scalar derive (#909)

#### Why are we making this change?

It was reported [here](#908)
that `rust_decimal::Decimal` errored out when `derive(cynic::Scalar)`
was used on it. This turned out to be because that `Decimal` type
defines its own `serialize` function in addition to implementing
`Serialize` and the `Scalar` derive was accidentally using that.

#### What effects does this change have?

Updaates the `Scalar` derive to specifically use
`serde::Serialize::serialize`

cynic-codegen/src/scalar_derive/mod.rs

@@ -67,7 +67,7 @@ pub fn scalar_derive_impl(input: ScalarDeriveInput) -> Result<TokenStream, syn::
             where
                 __S: cynic::serde::Serializer,
             {
-                self.0.serialize(serializer)
+                <#inner_type as cynic::serde::Serialize>::serialize(&self.0, serializer)
             }
         }
 

cynic-codegen/src/scalar_derive/snapshots/cynic_codegen__scalar_derive__tests__snapshot_scalar_derive.snap

@@ -8,7 +8,7 @@ impl cynic::serde::Serialize for DateTime {
     where
         __S: cynic::serde::Serializer,
     {
-        self.0.serialize(serializer)
+        <String as cynic::serde::Serialize>::serialize(&self.0, serializer)
     }
 }
 #[automatically_derived]