chore: Use empty headers when there is no value in builder.

Author: nstdio

src/main/java/io/github/nstdio/http/ext/Headers.java

@@ -51,7 +51,7 @@ class Headers {
   static final String HEADER_LAST_MODIFIED = "Last-Modified";
   static final String HEADER_WARNING = "Warning";
   static final BiPredicate<String, String> ALLOW_ALL = (s, s2) -> true;
-  private static final HttpHeaders EMPTY_HEADERS = HttpHeaders.of(Map.of(), ALLOW_ALL);
+  static final HttpHeaders EMPTY_HEADERS = HttpHeaders.of(Map.of(), ALLOW_ALL);
   private static final DateTimeFormatter ASCTIME_DATE_TIME = new DateTimeFormatterBuilder()
       .appendPattern("EEE MMM")
       .appendLiteral(' ')

src/main/java/io/github/nstdio/http/ext/HttpHeadersBuilder.java

@@ -24,6 +24,7 @@
 import java.util.function.BiPredicate;
 
 import static io.github.nstdio.http.ext.Headers.ALLOW_ALL;
+import static io.github.nstdio.http.ext.Headers.EMPTY_HEADERS;
 
 class HttpHeadersBuilder {
   private final TreeMap<String, List<String>> headersMap;
@@ -47,7 +48,7 @@ private void copyTo(HttpHeadersBuilder builder, Map<String, List<String>> source
   private List<String> values(String name, int capacity) {
     return headersMap.computeIfAbsent(name, k -> new ArrayList<>(capacity));
   }
-  
+
   private List<String> values(String name) {
     return values(name, 1);
   }
@@ -118,7 +119,7 @@ HttpHeaders build() {
   }
 
   HttpHeaders build(BiPredicate<String, String> filter) {
-    return HttpHeaders.of(headersMap, filter);
+    return headersMap.isEmpty() ? EMPTY_HEADERS : HttpHeaders.of(headersMap, filter);
   }
 
   @Override

src/test/kotlin/io/github/nstdio/http/ext/HttpHeadersBuilderTest.kt

@@ -16,6 +16,8 @@
 package io.github.nstdio.http.ext
 
 import io.github.nstdio.http.ext.Assertions.assertThat
+import io.kotest.matchers.maps.shouldBeEmpty
+import io.kotest.matchers.types.shouldBeSameInstanceAs
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
 
@@ -126,4 +128,15 @@ internal class HttpHeadersBuilderTest {
     assertThat(builder.build())
       .isEmpty()
   }
+
+  @Test
+  fun `Should not create new empty instances`() {
+    //when
+    val h1 = builder.build()
+    val h2 = builder.build()
+
+    //then
+    h1 shouldBeSameInstanceAs h2
+    h1.map().shouldBeEmpty()
+  }
 }