```ruby
require "spec_helper"

describe BSON::MinKey do

  describe "#__bson_encode__" do

    let(:min_key) do
      described_class.new
    end

    let(:buffer) do
      BSON::Buffer.new
    end

    let(:encoded) do
      min_key.__bson_encode__("key", buffer)
    end

    it "returns the buffer" do
      expect(encoded).to eq(buffer)
    end

    it "returns the same buffer instance" do
      expect(encoded).to eql(buffer)
    end

    it "encodes the field/min key pair to the buffer" do
      expect(encoded.bytes).to eq("\xFFkey\x00")
    end
  end
end
```

Fix ruby head encoding issue
user
```ruby
require "spec_helper"

describe BSON::MinKey do

  describe "#__bson_encode__" do

    let(:min_key) do
      described_class.new
    end

    let(:buffer) do
      BSON::Buffer.new
    end

    let(:encoded) do
      min_key.__bson_encode__("key", buffer)
    end

    it "returns the buffer" do
      expect(encoded).to eq(buffer)
    end

    it "returns the same buffer instance" do
      expect(encoded).to eql(buffer)
    end

    it "encodes the field/min key pair to the buffer" do
      expect(encoded.bytes).to eq("#{BSON::Types::MIN_KEY}key\x00")
    end
  end
end
```
assistant