Use invariant culture for converting XPath objects in tests

Before, a call to Convert.ChangeType() in the XPath tests used the current culture,
which resulted in test failures on cultures like de-AT and on Windows 10.
Instead, we now use the invariant culture when converting objects to fix this.

Fixes #136
Merge pull request #205 from akoeplinger/fix-xpath-tests

Use invariant culture for converting XPath objects in tests