Add addHeaders() function

Author: svenjacobs

firebase-functions/src/androidTest/java/com/google/firebase/functions/CallTests.kt

@@ -98,10 +98,12 @@ class CallTests {
       .addHeader("Header1", "value1")
       .addHeader("Header2", "value2")
       .addHeader("Header1", "value3")
+      .addHeaders(mapOf("Header3" to "value4"))
     val actual = Tasks.await(function.call()).getData() as? Map<*, *>
 
     assertThat(actual).isNotNull()
     assertThat(actual?.get("Header1")).isEqualTo("value3")
     assertThat(actual?.get("Header2")).isEqualTo("value2")
+    assertThat(actual?.get("Header3")).isEqualTo("value4")
   }
 }

firebase-functions/src/main/java/com/google/firebase/functions/HttpsCallableReference.kt

@@ -185,4 +185,16 @@ public class HttpsCallableReference {
     headers[name] = value
     return this
   }
+
+  /**
+   * Adds all HTTP headers of passed map for calls from this instance of Functions.
+   *
+   * Note that an existing header with the same name will be overwritten.
+   *
+   * @param headers Map of HTTP headers (name to value)
+   */
+  public fun addHeaders(headers: Map<String, String>): HttpsCallableReference {
+    this.headers.putAll(headers)
+    return this
+  }
 }