Add IO#set_encoding_by_bom method

Method was introduced in MRI in 2.7.0 in e717d6faa

spec/ruby/core/io/set_encoding_by_bom_spec.rb

@@ -67,5 +67,11 @@
 
       -> { @io.set_encoding_by_bom }.should raise_error(ArgumentError, 'encoding is set to UTF-8 already')
     end
+
+    it 'returns exception if encoding conversion is already set' do
+      @io.set_encoding(nil, Encoding::UTF_8)
+
+      -> { @io.set_encoding_by_bom }.should raise_error(ArgumentError, 'encoding conversion is set')
+    end
   end
 end

src/main/ruby/truffleruby/core/io.rb

@@ -2158,6 +2158,25 @@ def set_encoding(external, internal=nil, options=undefined)
     self
   end
 
+  def set_encoding_by_bom
+    unless binmode?
+      raise ArgumentError, 'ASCII incompatible encoding needs binmode'
+    end
+
+    if internal_encoding
+      raise ArgumentError, 'encoding conversion is set'
+    end
+
+    if external_encoding && external_encoding != Encoding::ASCII_8BIT
+      raise ArgumentError, "encoding is set to #{external_encoding} already"
+    end
+
+    external = strip_bom
+    if external
+      @external = Encoding.find(external)
+    end
+  end
+
   private def strip_bom
     mode = Truffle::POSIX.truffleposix_fstat_mode(Primitive.io_fd(self))
     return unless Truffle::StatOperations.file?(mode)