fix: allow empty HTTP header values

modules/java-http-client/src/java/net/http/HttpHeaders.scala

@@ -65,7 +65,6 @@ object HttpHeaders:
 
       requireNonNull(key)
       requireNonNull(values, s"values for key '${key}' can not be null")
-      require(!values.isEmpty, s"empty values for key '${key}'")
 
       val headerKey = key.trim()
       require(!headerKey.isEmpty, "empty key")
@@ -92,7 +91,6 @@ object HttpHeaders:
       values.forEach { s =>
         requireNonNull(s, s"header value can not be null")
         val trimedValue = requireNonNull(s).trim()
-        require(!trimedValue.isEmpty, s"empty value for key '${headerKey}'")
 
         if (filter.test(headerKey, trimedValue))
           headerValues.add(trimedValue): Unit

modules/java-http-client/test/src/java/net/http/EmptyHeaderValuesTest.scala

@@ -0,0 +1,28 @@
+package snhttp.test.java.net.http
+
+import java.net.http.HttpHeaders
+import java.util.{List as JList, Map as JMap}
+import utest.{TestSuite, Tests, test, assert}
+
+class EmptyHeaderValuesTest extends TestSuite:
+  val tests = Tests:
+    test("empty header value is retained") {
+      val headers = HttpHeaders.of(JMap.of("X-Empty", JList.of("")), (_, _) => true)
+      assert(headers.firstValue("X-Empty").isPresent())
+      assert(headers.firstValue("X-Empty").get() == "")
+    }
+    test("whitespace header value becomes an empty value") {
+      val headers = HttpHeaders.of(JMap.of("X-Empty", JList.of("  \t ")), (_, _) => true)
+      assert(headers.allValues("X-Empty") == JList.of(""))
+    }
+    test("empty header value list is omitted") {
+      val headers = HttpHeaders.of(JMap.of("X-Empty", JList.of[String]()), (_, _) => true)
+      assert(headers.map().isEmpty())
+    }
+    test("filter can reject empty values while retaining other values") {
+      val headers = HttpHeaders.of(
+        JMap.of("X-Values", JList.of("", "  value  ", " ")),
+        (_, value) => !value.isEmpty(),
+      )
+      assert(headers.allValues("X-Values") == JList.of("value"))
+    }