I think Array#reduce should not modify array object

original = ['fall','aff','les']
Test.assert_equals( original.reduce(sum), 'fallaffles')
Test.assert_equals( original, ['fall','aff','les'])

I think

Test.assert_equals(always().(),nil , 'if n is undefined then executing the return value of always is nil')

could be

Test.assert_equals(always(nil).(),nil , 'if n is nil then executing the return value of always is nil')

should test more invalid cases.

Test.assert_equals(get_hashtags("two hashs##in middle of word#"), [])