Add headerBuilder.addAll(headers) method (#4121)

okhttp-tests/src/test/java/okhttp3/HeadersTest.java

@@ -343,6 +343,20 @@ public final class HeadersTest {
     assertEquals("A: a\nB: bb\n", headers.toString());
   }
 
+  @Test public void headersAddAll() {
+    Headers sourceHeaders = new Headers.Builder()
+        .add("A", "aa")
+        .add("a", "aa")
+        .add("B", "bb")
+        .build();
+    Headers headers = new Headers.Builder()
+        .add("A", "a")
+        .addAll(sourceHeaders)
+        .add("C", "c")
+        .build();
+    assertEquals("A: a\nA: aa\na: aa\nB: bb\nC: c\n", headers.toString());
+  }
+
   /** See https://github.com/square/okhttp/issues/2780. */
   @Test public void testDigestChallenges() {
     // Strict RFC 2617 header.

okhttp/src/main/java/okhttp3/Headers.java

@@ -283,12 +283,26 @@ public Builder add(String line) {
       return add(line.substring(0, index).trim(), line.substring(index + 1));
     }
 
-    /** Add a field with the specified value. */
+    /**
+     * Add a header with the specified name and value. Does validation of header names and values.
+     */
     public Builder add(String name, String value) {
       checkNameAndValue(name, value);
       return addLenient(name, value);
     }
 
+    /**
+     * Adds all headers from an existing collection. Does validation of header names and values.
+     */
+    public Builder addAll(Headers headers) {
+      int size = headers.size();
+      for (int i = 0; i < size; i++) {
+        addLenient(headers.name(i), headers.value(i));
+      }
+
+      return this;
+    }
+
     /**
      * Add a field with the specified value without any validation. Only appropriate for headers
      * from the remote peer or cache.