Don't send empty pairs

If you call an operation with a hash that contains keys in some records and doesn't in others, add_batch would add an empty key like this:

<sObject>
<id>.....</id>
<Outdoor_Temp__c>57.2</Outdoor_Temp__c>
<Temp_Not_Available__c>false</Temp_Not_Available__c>
<Wind_Direction_degrees__c/>
<Wind_Speed_mph__c/>
</sObject>

This makes SFDC throw up when a field has a type of Number.  This pull eliminates adding the key if the value is .blank?.