ruby 1.8.7 compat: starts/ends_with? doesn't cast to string

`starts/ends_with?` methods shouldn't cast argument to string because
ruby 1.8.7 doesn't seem to do that. for example:

    "foobar".ends_with?(:bar)
    # => true in ActiveSupport implementation, false in ruby 1.8.7

[#3199 state:committed]

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

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

@@ -20,14 +20,12 @@ def self.append_features(base)
 
         # Does the string start with the specified +prefix+?
         def starts_with?(prefix)
-          prefix = prefix.to_s
           self[0, prefix.length] == prefix
         end
 
         # Does the string end with the specified +suffix+?
         def ends_with?(suffix)
-          suffix = suffix.to_s
-          self[-suffix.length, suffix.length] == suffix      
+          self[-suffix.length, suffix.length] == suffix
         end
       end
     end

activesupport/test/core_ext/string_ext_test.rb

@@ -177,6 +177,7 @@ def test_starts_ends_with_alias
     s = "hello"
     assert s.starts_with?('h')
     assert s.starts_with?('hel')
+    assert !s.starts_with?(:hel)
     assert !s.starts_with?('el')
 
     assert s.start_with?('h')
@@ -185,6 +186,7 @@ def test_starts_ends_with_alias
 
     assert s.ends_with?('o')
     assert s.ends_with?('lo')
+    assert !s.ends_with?(:lo)
     assert !s.ends_with?('el')
 
     assert s.end_with?('o')