Error 415 while Updating using OData.

Gurus,

I am trying to update Asset using BAPI  BAPI_FIXEDASSET_CHANGE.

I tried to follow the following link in SCN.

Detailed step by step procedure for Creating Gateway Service with all the CRUD Operations and testing them in Service Explorer Part1

But for some reason I wasn't successful, so what I did was I created the custom RFC and Passed 'X' in all necessary places (to whichever fields I needed to update) and used that RFC in ASSETCHANGESET_UPDATE_ENTITY

  Have also written codes to get entries in Read and Query Operations in ASSETCHANGESET_GET_ENTITY and ASSETCHANGESET_GET_ENTITYSET respectively.

Now finally when I dry-run my Odata Query runs fine and I then I select Use as request button and get XML code and then I change the necessary fields and then select POST-> F8, error 415. What did I do wrong?

BTW the XML generated by Use as request button doesn't start typically by <?xml version="1.0" encoding="utf-8" standalone="yes"?>

It starts with  <feed xmlns="http://www.w3.org/2005/Atom" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata" xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices" xml:base="The Service name">.

I am new to Netweaver Gateway, so, I kindly urge you to explain me in detail. Thanks.