- Make sure that input stream is seekable

I guess I'm not using a pattern which prevents my current issue but when I'm registering sprite maps with

```
CCSpriteFrameCache.AddSpriteFramesWithFile
```

I get an exception on Android. The plist file is opened via TitleContainer.OpenStream and this returns a non-seekable stream.

```
Microsoft.Xna.Framework.TitleContainer.OpenStream (name="Content/tiles/tiles.plist") in ...cocos2d-xna/MonoGame/MonoGame.Framework/TitleContainer.cs:144
Cocos2D.CCContentManager.InternalLoad<Cocos2D.PlistDocument> (assetName="tiles/tiles.plist", path="tiles/tiles.plist", weakReference=false) in ...cocos2d-xna/cocos2d/Platform/CCContentManager.cs:302
Cocos2D.CCContentManager.Load<Cocos2D.PlistDocument> (assetName="tiles/tiles.plist", weakReference=false) in ...cocos2d-xna/cocos2d/Platform/CCContentManager.cs:230
Cocos2D.CCContentManager.Load<Cocos2D.PlistDocument> (assetName="tiles/tiles.plist") in ...cocos2d-xna/cocos2d/Platform/CCContentManager.cs:185
Cocos2D.CCSpriteFrameCache.AddSpriteFramesWithFile (pszPlist="tiles/tiles.plist") in ...cocos2d-xna/cocos2d/sprite_nodes/CCSpriteFrameCache.cs:192
```

Local vars in PlistDocument looks like this:

```
public void LoadFromXmlFile(Stream data)

this              	{Cocos2D.PlistDocument}                                     	Cocos2D.PlistDocument
data              	{Android.Runtime.InputStreamInvoker}                        	Android.Runtime.InputStreamInvoker
base              	{System.IO.Stream}                                          	System.IO.Stream
BaseInputStream   	{android.content.res.AssetManager$AssetInputStream@4142cb70}	Java.IO.InputStreamInvoker
CanRead           	true                                                        	bool
CanSeek           	false                                                       	bool
CanWrite          	false                                                       	bool
Length            	System.NotSupportedException: Operation is not supported.   	
Position          	System.NotSupportedException: Operation is not supported.   	
Non-public members	                                                            	
magicHeader       	(null)                                                      	System.Byte[]
```

Simple fix is attached.