change byte_slice(Int) ArgumentError->IndexError when out of range

spec/std/string_spec.cr

@@ -229,6 +229,18 @@ describe "String" do
     it "gets byte_slice with negative index" do
       "hello".byte_slice(-2, 3).should eq("lo")
     end
+
+    it "gets byte_slice(Int) with with start out of bounds" do
+      expect_raises(IndexError) do
+        "hello".byte_slice(10)
+      end
+    end
+
+    it "gets byte_slice(Int) with with start out of bounds" do
+      expect_raises(IndexError) do
+        "hello".byte_slice(-10)
+      end
+    end
   end
 
   describe "to_i" do

src/string.cr

@@ -953,8 +953,10 @@ class String
     end
   end
 
-  def byte_slice(start : Int)
-    byte_slice start, bytesize - start
+  def byte_slice(start : Int) : String
+    count = bytesize - start
+    raise IndexError.new if start > 0 && count < 0
+    byte_slice start, count
   end
 
   # Returns a substring starting from the *start* byte.