Correct Regexp extension un/optionalize assertions and fix failures masked by false positives

This patch corrects the Regexp extension un/optionalize test assertions. Previously, assert was being used instead of assert_equal, producing false positives.

-  assert "", Regexp.optionalize("")
+  assert_equal "", Regexp.optionalize("")

The first will always pass since "" evaluates to true. It's clear that the original author intended assert_equal.

Fixing the assertions uncovered a failure which this patch also fixes:

test_optionalize(RegexpExtAccessTests)
    [./test/core_ext/regexp_ext_test.rb:18:in `test_optionalize'
<""> expected but was
<"(?:)?">.

Since the previous return value of /(?:)?/ will still match an empty string, behavior is unchanged (all Action Pack tests continue to pass), but based on the tests, "" was the return value the author intended.