Encoding: compare with case insensitive UTF-8 to avoid using iconv wh…

…en not necessary

spec/std/io/io_spec.cr

@@ -689,6 +689,12 @@ describe IO do
         end
       end
 
+      it "sets encoding to utf-8 and stays as UTF-8" do
+        io = SimpleIOMemory.new(Base64.decode_string("ey8qx+Tl8fwg7+Dw4Ozl8vD7IOLo5+jy4CovfQ=="))
+        io.set_encoding("utf-8")
+        io.encoding.should eq("UTF-8")
+      end
+
       it "does skips when converting to UTF-8" do
         io = SimpleIOMemory.new(Base64.decode_string("ey8qx+Tl8fwg7+Dw4Ozl8vD7IOLo5+jy4CovfQ=="))
         io.set_encoding("UTF-8", invalid: :skip)

src/io.cr

@@ -1023,7 +1023,7 @@ abstract class IO
   # String operations (`gets`, `gets_to_end`, `read_char`, `<<`, `print`, `puts`
   # `printf`) will use this encoding.
   def set_encoding(encoding : String, invalid : Symbol? = nil)
-    if (encoding == "UTF-8") && (invalid != :skip)
+    if encoding.compare("UTF-8", case_insensitive: true) == 0 && invalid != :skip
       @encoding = nil
     else
       @encoding = EncodingOptions.new(encoding, invalid)