option binding problem with p:add-attribute

[Conal-Tuohy]

When attempting to add an attribute with a value taken from a variable, if the variable's name is $match, then the value of the p:add-attribute's match attribute is used, instead of the variable.

<p:declare-step version="1.0" name="test" xmlns:p="http://www.w3.org/ns/xproc">
    <p:output port="result"/>
    <p:variable name="match" select=" 'test' "/>
    <p:identity>
        <p:input port="source">
            <p:inline>
                <example/>
            </p:inline>
        </p:input>
    </p:identity>
    <p:add-attribute match="/*" attribute-name="example">
        <p:with-option name="attribute-value" select="$match"/>
    </p:add-attribute>
</p:declare-step>

Result:

<example example="/*"/>

Expected:

<example example="test"/>

(using xmlcalabash-1.1.4-95)