Use dedicated XmlComponent rather than polute pure one

src/file/footer/footer.ts

@@ -1,10 +1,10 @@
 // http://officeopenxml.com/WPfooters.php
-import { XmlComponent } from "file/xml-components";
+import { InitializableXmlComponent, XmlComponent } from "file/xml-components";
 import { Paragraph } from "../paragraph";
 import { Table } from "../table";
 import { FooterAttributes } from "./footer-attributes";
 
-export class Footer extends XmlComponent {
+export class Footer extends InitializableXmlComponent {
     private readonly refId: number;
 
     constructor(referenceNumber: number, initContent?: XmlComponent) {

src/file/header/header.ts

@@ -1,10 +1,10 @@
 // http://officeopenxml.com/WPheaders.php
-import { XmlComponent } from "file/xml-components";
+import { InitializableXmlComponent, XmlComponent } from "file/xml-components";
 import { Paragraph } from "../paragraph";
 import { Table } from "../table";
 import { HeaderAttributes } from "./header-attributes";
 
-export class Header extends XmlComponent {
+export class Header extends InitializableXmlComponent {
     private readonly refId: number;
 
     constructor(referenceNumber: number, initContent?: XmlComponent) {

src/file/xml-components/index.ts

@@ -3,3 +3,4 @@ export * from "./attributes";
 export * from "./default-attributes";
 export * from "./imported-xml-component";
 export * from "./xmlable-object";
+export * from "./initializable-xml-component";

src/file/xml-components/initializable-xml-component.ts

@@ -0,0 +1,11 @@
+import { XmlComponent } from "file/xml-components";
+
+export abstract class InitializableXmlComponent extends XmlComponent {
+    constructor(rootKey: string, initComponent?: InitializableXmlComponent) {
+        super(rootKey);
+
+        if (initComponent) {
+            this.root = initComponent.root;
+        }
+    }
+}

src/file/xml-components/xml-component.ts

@@ -5,9 +5,9 @@ export { BaseXmlComponent };
 export abstract class XmlComponent extends BaseXmlComponent {
     protected root: Array<BaseXmlComponent | string>;
 
-    constructor(rootKey: string, initContent?: XmlComponent) {
+    constructor(rootKey: string) {
         super(rootKey);
-        this.root = initContent ? initContent.root : new Array<BaseXmlComponent>();
+        this.root = new Array<BaseXmlComponent>();
     }
 
     public prepForXml(): IXmlableObject {