Clearer String#first and #last edge cases. Fix that foo.first(0) == i…

…nstead of foo.

activesupport/lib/active_support/core_ext/string/access.rb

@@ -41,17 +41,29 @@ def to(position)
           #   "hello".first(2)  # => "he"
           #   "hello".first(10) # => "hello"
           def first(limit = 1)
-            mb_chars[0..(limit - 1)].to_s
+            if limit == 0
+              ''
+            elsif limit >= size
+              self
+            else
+              mb_chars[0...limit].to_s
+            end
           end
-          
+
           # Returns the last character of the string or the last +limit+ characters.
           #
           # Examples: 
           #   "hello".last     # => "o"
           #   "hello".last(2)  # => "lo"
           #   "hello".last(10) # => "hello"
           def last(limit = 1)
-            (mb_chars[(-limit)..-1] || self).to_s
+            if limit == 0
+              ''
+            elsif limit >= size
+              self
+            else
+              mb_chars[(-limit)..-1].to_s
+            end
           end
         end
       else

activesupport/test/core_ext/string_ext_test.rb

@@ -132,10 +132,12 @@ def test_access
 
     assert_equal "h", s.first
     assert_equal "he", s.first(2)
+    assert_equal "", s.first(0)
 
     assert_equal "o", s.last
     assert_equal "llo", s.last(3)
     assert_equal "hello", s.last(10)
+    assert_equal "", s.last(0)
 
     assert_equal 'x', 'x'.first
     assert_equal 'x', 'x'.first(4)