fix: Consider an empty string header value to be present

Sources/SmithyHTTPAPI/Headers.swift

@@ -138,15 +138,7 @@ public struct Headers: Sendable {
     }
 
     public func exists(name: String) -> Bool {
-        guard headers.index(of: name) != nil else {
-            return false
-        }
-
-        guard let value = value(for: name) else {
-            return false
-        }
-
-        return !value.isEmpty
+        headers.index(of: name) != nil
     }
 
     /// The dictionary representation of all headers.

Tests/ClientRuntimeTests/NetworkingTests/Http/HttpHeadersTests.swift → Tests/SmithyHTTPAPITests/HeadersTests.swift

@@ -1,13 +1,14 @@
-/*
- * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
- * SPDX-License-Identifier: Apache-2.0.
- */
+//
+// Copyright Amazon.com Inc. or its affiliates.
+// All Rights Reserved.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
 
 import SmithyHTTPAPI
 import XCTest
-import ClientRuntime
 
-class HttpHeadersTests: XCTestCase {
+class HeadersTests: XCTestCase {
 
     var headersAsDictionaryWithArray = [String: [String]]()
     var headersAsDictionary = [String: String]()
@@ -123,4 +124,21 @@ class HttpHeadersTests: XCTestCase {
         XCTAssertEqual(headerA, headerB)
         XCTAssertEqual(headerA.hashValue, headerB.hashValue)
     }
+
+    // MARK: - exists()
+
+    func test_exists_trueIfHeaderExists() {
+        let subject = Headers(["a": "abc"])
+        XCTAssertTrue(subject.exists(name: "a"))
+    }
+
+    func test_exists_falseIfHeaderDoesntExist() {
+        let subject = Headers(["a": "abc"])
+        XCTAssertFalse(subject.exists(name: "b"))
+    }
+
+    func test_exists_trueIfHeaderValueIsEmptyString() {
+        let subject = Headers(["a": ""])
+        XCTAssertTrue(subject.exists(name: "a"))
+    }
 }