Extra specs for String#append_as_bytes

Describe behaviour for negative numbers and numbers outside the byte
range.

core/string/append_as_bytes_spec.rb

@@ -34,6 +34,18 @@
       str.should == "hello\xE2\x82\f+\xAC".b
     end
 
+    it "truncates integers to the least significant byte" do
+      str = +""
+      str.append_as_bytes(0x131, 0x232, 0x333, bignum_value, bignum_value(1))
+      str.bytes.should == [0x31, 0x32, 0x33, 0, 1]
+    end
+
+    it "wraps negative integers" do
+      str = "".b
+      str.append_as_bytes(-1, -bignum_value, -bignum_value(1))
+      str.bytes.should == [0xFF, 0, 0xFF]
+    end
+
     it "only accepts strings or integers, and doesn't attempt to cast with #to_str or #to_int" do
       to_str = mock("to_str")
       to_str.should_not_receive(:to_str)