Test correct handling of whitespace in Ruleset tests

[hayfield]

As noted at #229 (comment):

The XPath spec which shows that text() returns the entire text node. The XPath in the mapping file says that the comparison is against this value (rather than a trimmed or stripped version). As such, checking against a modified text node is not correct.
Further, the XML Spec shows that white space is significant and must be treated as part of a value - treating it as not-present is incorrect.

As such, any Rule that undertakes checks on strings should have tests to ensure whitespace is included in the check (rather than being stripped, ignored, or such).

This covers the following standard Rules:

• sum (whitespace currently tested in valid_sum.xml)
• date_order - (no whitespace tests at present - attributes in the Standard Ruleset are covered by Schema checks for a correct date type (no whitespace), though other attributes or element text are not)
• regex_matches - (can see no tests at present, though understand whitespace may currently be ignored due to an interpretation of perl style regular expression)
• regex_no_matches - (can see no tests at present, though understand whitespace may currently be ignored due to an interpretation of perl style regular expression)
• startswith - (no whitespace tests at present)
• unique - (no whitespace tests at present)