Fix HTTPBody length calculation for utf8 strings. (#60)

This PR aims to fix the byte length calculation for utf8 strings.

Author: tib

Sources/OpenAPIRuntime/Interface/HTTPBody.swift

@@ -142,7 +142,7 @@ public final class HTTPBody: @unchecked Sendable {
     public let iterationBehavior: IterationBehavior
 
     /// Describes the total length of the body, if known.
-    public enum Length: Sendable {
+    public enum Length: Sendable, Equatable {
 
         /// Total length not known yet.
         case unknown
@@ -542,10 +542,7 @@ extension HTTPBody {
     @inlinable public convenience init(
         _ string: some StringProtocol & Sendable
     ) {
-        self.init(
-            ByteChunk(string),
-            length: .known(string.count)
-        )
+        self.init(ByteChunk(string))
     }
 
     /// Creates a new body with the provided async throwing stream of strings.

Tests/OpenAPIRuntimeTests/Interface/Test_HTTPBody.swift

@@ -185,6 +185,13 @@ final class Test_Body: Test_Runtime {
         XCTAssertEqual(chunks, ["hel", "lo"].map { Array($0.utf8)[...] })
     }
 
+    func testUTF8String() async throws {
+        XCTAssertEqual(HTTPBody("abc").length, .known(3))
+        XCTAssertEqual(HTTPBody("🤘").length, .known(4))
+        XCTAssertEqual(HTTPBody("\u{1f603}").length, .known(4))
+        XCTAssertEqual(HTTPBody("árvíztűrő tükörfúrógép").length, .known(31))
+    }
+
     func testIterationBehavior_single() async throws {
         let sequence = AsyncStream(
             String.self,