XMLUnit.NET 2.7.1

The release fixes a bug in a corner-case and adds a new isNumber placeholder.

The full list of changes:

• add a new ${xmlunit.isNumber} placeholder
  Based on the Java PR xmlunit/#154 by @NathanAtClarity.

• the XPath values of a comparison should not be affected by any NodeFilter being in effect.
  Issue similar to xmlunit/#156