Probably not the answer. but you can set the expect100Continue value to your App.Config. Setting this will be applied to the all web service call. you don't need to set it on each webservice call separately.
put it inside Configuration->system.net->settings
<system.net>
<settings>
<servicePointManager expect100Continue="false"/>
</settings>
</system.net>
Note: Do not change this settings unless you decided to ignore the TCP 100Continue response result.