Update is broken with error: INVALID_TYPE: Must send a concrete entity type.

[skplunkerin]

No matter how I call the update or update! methods, I get the following error:

INVALID_TYPE: Must send a concrete entity type.

Doing some research, it's because you're setting the sobject_type wrong.

The final XML should look something like this:
NOTE the tns:sObjects xsi:type attribute that holds the sobject_type of "Account"

(see the Update() API Calls section (Scenario 7))

<tns:update>
    <tns:sObjects xsi:type="urn1:Account">
        <tns:Id>006A000000Lbiiz</tns:Id>
        <tns:Name>Whizbang Corp</tns:Name>
    </tns:sObjects>
</tns:update>

Not this (which is what it's doing):

<tns:update>
    <tns:sObjects>
        <ins0:type>Account</ins0:type>
        <tns:Id>006A000000Lbiiz</tns:Id>
        <tns:Name>Whizbang Corp</tns:Name>
    </tns:sObjects>
</tns:update>

[skplunkerin]

Looks like this is fixed if you use xsi:type instead of ins0:type, by changing the following within the sobjects_hash() method:

sobjects.map! do |obj|
    {"ins0:type" => sobject_type}.merge(obj)
end

to:

sobjects.map! do |obj|
    {"@xsi:type" => sobject_type}.merge(obj)
end

I'll create a PR that includes this fix.