Reject underscored XPath function names

[sjh9714]

Summary

Fixes #352.

Reject XPath function names containing underscores instead of treating them as
aliases for hyphenated standard functions. Standard names such as
local-name() continue to parse normally.

Changes

• Stop underscored names from being parsed as XPath function calls.
• Update XPath fixtures to use standard hyphenated names and add regression
  coverage.

Testing

• ruby -Itest -Ilib test/parser/test_xpath.rb --name=test_function_with_underscore
• rake test

[kou]

I want to keep backward compatibility as much as possible.

How about rejecting _ only when a user specify an option explicitly?

[sjh9714]

Updated this so underscored function names remain accepted by default and are rejected only when the existing strict: true option is passed. The low-level XPath parser now receives that option, and the tests cover both default compatibility and strict rejection.

Ran:

• ruby -Ilib:test test/parser/test_xpath.rb
• ruby -Ilib:test test/functions/test_base.rb
• ruby -Ilib:test test/run.rb