expand variables in value attributes too

[halex2005]

Hi!

I found an issue for expand abstract patterns.

I need to declare abstract pattern which uses external function inside (I use custom extensibility functions for .NET XslCompiledTransform) and I need to pass external parameter to this function. Then I need to assert function result multiple times.

Here is pseudocode:

<schema xmlns="http://purl.oclc.org/dsdl/schematron">
  <ns prefix="extension" uri="urn:extension" />

  <!-- declare abstract pattern -->
  <pattern abstract="true" id="abstract-pattern">
    <rule context="$context">
      <let name="someVariable" value="extension:function(@attribute, $externalParameter)" />
      <assert test="normalize-space($someVariable) = 'someValue1' ">
        error message
      </assert>
      <report test="normalize-space($someVariable) = 'someValue2' ">
        report message
      </report>
    </rule>
  </pattern>

  <!-- use abstract pattern -->
  <pattern is-a="abstract-pattern" id="concrete-pattern">
    <param name="$context" value="/*" />
    <param name="$externalParameter" value="999" />
  </pattern>
</schema>

The issue is that iso_abstract_expand.xsl does not substitute parameters for value attributes.

Please, consider this PR which fixes this behavior.

[tgraham-antenna]

Fixes same problem as #57.

Related issue: #5