fix(lib): Fixed typos in multiple modules

BREAKING CHANGE: The `Preference` header had a typo in a variant and it's string representation,
  change `Preference::HandlingLeniant` to `Preference::HandlingLenient`.

src/header/mod.rs

@@ -476,7 +476,7 @@ impl Headers {
     ///
     /// If a header already contains a value, this will add another line to it.
     ///
-    /// If a header doesnot exist for this name, a new one will be created with
+    /// If a header does not exist for this name, a new one will be created with
     /// the value.
     ///
     /// Example: