Fix: Honour encoding in IO::Memory#to_s (#11875)

spec/std/io/memory_spec.cr

@@ -61,6 +61,37 @@ describe IO::Memory do
       # Ensure that the buffer is resized, otherwise the spec doesn't work
       io.@capacity.should_not eq old_capacity
     end
+
+    {% if flag?(:without_iconv) %}
+      pending "encoding"
+    {% else %}
+      describe "encoding" do
+        it "returns String" do
+          io = IO::Memory.new
+          io.set_encoding "UTF-16LE"
+          io << "abc"
+          io.to_s.should eq "abc"
+          io.to_slice.should eq Bytes[0x61, 0, 0x62, 0, 0x63, 0]
+        end
+
+        it "writes to IO" do
+          io1 = IO::Memory.new
+          io1.set_encoding "UTF-32LE"
+
+          io2 = IO::Memory.new
+          io2.set_encoding "UTF-16LE"
+
+          io1.write_utf8 "abc😂".to_slice
+          io1.to_s io2
+          byte_slice = io2.to_slice
+          utf16_slice = Slice.new(byte_slice.to_unsafe.unsafe_as(Pointer(UInt16)), byte_slice.size // sizeof(UInt16))
+
+          String.from_utf16(utf16_slice).should eq "abc😂"
+          byte_slice.should eq Bytes[0x61, 0, 0x62, 0, 0x63, 0, 0x3D, 0xD8, 0x02, 0xDE]
+          utf16_slice.should eq Slice[0x0061, 0x0062, 0x0063, 0xD83D, 0xDE02]
+        end
+      end
+    {% end %}
   end
 
   it "reads single line content" do

src/io/memory.cr

@@ -404,7 +404,15 @@ class IO::Memory < IO
   # io.to_s # => "123"
   # ```
   def to_s : String
-    String.new @buffer, @bytesize
+    if encoding = @encoding
+      {% if flag?(:without_iconv) %}
+        raise NotImplementedError.new("String.encode")
+      {% else %}
+        String.new to_slice, encoding: encoding.name, invalid: encoding.invalid
+      {% end %}
+    else
+      String.new @buffer, @bytesize
+    end
   end
 
   # Returns the underlying bytes.
@@ -427,7 +435,15 @@ class IO::Memory < IO
       new_bytesize = bytesize * 2
       resize_to_capacity(new_bytesize) if @capacity < new_bytesize
     end
-    io.write(to_slice)
+    if encoding = @encoding
+      {% if flag?(:without_iconv) %}
+        raise NotImplementedError.new("String.encode")
+      {% else %}
+        String.encode(to_slice, encoding.name, io.encoding, io, io.@encoding.try(&.invalid))
+      {% end %}
+    else
+      io.write(to_slice)
+    end
   end
 
   private def check_writeable

src/string.cr

@@ -1655,7 +1655,7 @@ class String
   end
 
   # :nodoc:
-  protected def self.encode(slice, from, to, io, invalid)
+  def self.encode(slice, from, to, io, invalid)
     IO::EncodingOptions.check_invalid(invalid)
 
     inbuf_ptr = slice.to_unsafe