Fix String#sub with negative index

spec/std/string_spec.cr

@@ -1173,13 +1173,27 @@ describe "String" do
       string.bytesize.should eq("あいのえお".bytesize)
     end
 
+    it "subs at negative index with char" do
+      string = "abc".sub(-1, 'd')
+      string.should eq("abd")
+      string = string.sub(-2, 'n')
+      string.should eq("and")
+    end
+
     it "subs at index with string" do
       string = "hello".sub(1, "eee")
       string.should eq("heeello")
       string.bytesize.should eq(7)
       string.size.should eq(7)
     end
 
+    it "subs at negative index with string" do
+      string = "hello".sub(-1, "ooo")
+      string.should eq("hellooo")
+      string.bytesize.should eq(7)
+      string.size.should eq(7)
+    end
+
     it "subs at index with string, non-ascii" do
       string = "あいうえお".sub(2, "けくこ")
       string.should eq("あいけくこえお")

src/string.cr

@@ -1857,7 +1857,7 @@ class String
   end
 
   private def sub_index(index, replacement)
-    index += size + 1 if index < 0
+    index += size if index < 0
 
     byte_index = char_index_to_byte_index(index)
     raise IndexError.new unless byte_index