Fix FrozenError when using interactive environment selector

lib/tomo/console/key_reader.rb

@@ -10,7 +10,7 @@ class KeyReader
       extend Forwardable
 
       def initialize(input=$stdin)
-        @buffer = ""
+        @buffer = +""
         @input = input
       end
 

test/tomo/console/key_reader_test.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require "stringio"
+
+class Tomo::Console
+  class KeyReaderTest < Minitest::Test
+    def setup
+      @input = StringIO.new
+      @input.define_singleton_method(:raw) do |&block|
+        block.call
+      end
+    end
+
+    def test_reads_a_single_keystroke
+      @input << "h"
+      @input.rewind
+
+      key_reader = KeyReader.new(@input)
+      char = key_reader.next
+
+      assert_equal("h", char)
+    end
+  end
+end