Added option to override content type used for json updates.
Added option to override content type used for json updates.

Certain services requires specific settings for this content type,
e.g Microsoft Service Provider foundation requires it to be set to Application/json;odata=verbose

denstepa/ruby_odata,denstepa/ruby_odata,zzk/ruby_odata,zzk/ruby_odata,zzk/ruby_odata,visoft/ruby_odata,denstepa/ruby_odata,visoft/ruby_odata