fix: Do not duplicate headers when configured via builder.

Author: nstdio

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

@@ -52,8 +52,8 @@ private HttpRequest apply(HttpRequest request) {
   private Map<String, List<String>> addHeaders(HttpHeaders h) {
     var headersBuilder = new HttpHeadersBuilder(h);
 
-    headers.forEach(headersBuilder::add);
-    resolvableHeaders.forEach((name, valueSupplier) -> headersBuilder.add(name, valueSupplier.get()));
+    headers.forEach(headersBuilder::addIfNotExist);
+    resolvableHeaders.forEach((name, valueSupplier) -> headersBuilder.addIfNotExist(name, valueSupplier.get()));
 
     return headersBuilder.map();
   }

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

@@ -44,16 +44,31 @@ 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);
+  }
+
   HttpHeadersBuilder add(String name, String value) {
-    headersMap.computeIfAbsent(name, k -> new ArrayList<>(1))
-        .add(value);
+    values(name).add(value);
+    return this;
+  }
+
+  HttpHeadersBuilder addIfNotExist(String name, String value) {
+    List<String> existing = values(name);
+    if (!existing.contains(value)) {
+      existing.add(value);
+    }
+
     return this;
   }
 
   HttpHeadersBuilder add(String name, List<String> values) {
     if (!values.isEmpty()) {
-      headersMap.computeIfAbsent(name, k -> new ArrayList<>(values.size()))
-          .addAll(values);
+      values(name, values.size()).addAll(values);
     }
     return this;
   }

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

@@ -56,6 +56,31 @@ class HeadersAddingInterceptorTest {
     }
   }
 
+  @Test
+  fun `Should not duplicate headers`() {
+    //given
+    val interceptor = HeadersAddingInterceptor(
+      mapOf("a" to "1", "b" to "3"),
+      mapOf("b" to Supplier { "2" }),
+    )
+
+    val request = HttpRequest.newBuilder("https://example.com".toUri())
+      .header("a", "1")
+      .header("b", "2")
+      .header("b", "3")
+      .build()
+    val chain = Chain.of<Any>(RequestContext.of(request, bodyHandler))
+
+    //when
+    val actual = interceptor.intercept(chain).request().headers()
+
+    //then
+    actual.map().should { 
+      it.shouldContain("a", listOf("1"))
+      it.shouldContain("b", listOf("2", "3"))
+    }
+  }
+
   @Test
   fun `Should not add headers to the chain when response is present`() {
     //given