use char when replacing a single character

Reasons:
- More idiomatic use of string.

Considering that they are constants, I don't anticipate any performance
gains related to heap-allocation.

Signed-off-by: Perma Alesheikh <me@prma.dev>

src/email/message/arg/body.rs

@@ -12,7 +12,7 @@ pub struct MessageRawBodyArg {
 
 impl MessageRawBodyArg {
     pub fn raw(self) -> String {
-        self.raw.join(" ").replace("\r", "").replace("\n", "\r\n")
+        self.raw.join(" ").replace('\r', "").replace('\n', "\r\n")
     }
 }
 

src/email/message/arg/header.rs

@@ -12,7 +12,7 @@ pub struct HeaderRawArgs {
 }
 
 pub fn raw_header_parser(raw_header: &str) -> Result<(String, String), String> {
-    if let Some((key, val)) = raw_header.split_once(":") {
+    if let Some((key, val)) = raw_header.split_once(':') {
         Ok((key.trim().to_owned(), val.trim().to_owned()))
     } else {
         Err(format!("cannot parse raw header {raw_header:?}"))

src/email/message/arg/mod.rs

@@ -15,6 +15,6 @@ pub struct MessageRawArg {
 
 impl MessageRawArg {
     pub fn raw(self) -> String {
-        self.raw.join(" ").replace("\r", "").replace("\n", "\r\n")
+        self.raw.join(" ").replace('\r', "").replace('\n', "\r\n")
     }
 }

src/email/message/template/arg/body.rs

@@ -12,7 +12,7 @@ pub struct TemplateRawBodyArg {
 
 impl TemplateRawBodyArg {
     pub fn raw(self) -> String {
-        self.raw.join(" ").replace("\r", "")
+        self.raw.join(" ").replace('\r', "")
     }
 }
 

src/email/message/template/arg/mod.rs

@@ -13,6 +13,6 @@ pub struct TemplateRawArg {
 
 impl TemplateRawArg {
     pub fn raw(self) -> String {
-        self.raw.join(" ").replace("\r", "")
+        self.raw.join(" ").replace('\r', "")
     }
 }