Update to be frozen string literal safe

.github/workflows/ci.yml

@@ -30,6 +30,8 @@ jobs:
           bundler-cache: true
       - name: Run tests
         run: bundle exec rake test:units
+        env:
+          RUBYOPT: ${{ startsWith(matrix.ruby, 'head') && '--enable=frozen-string-literal' || '' }}
 
   test-legacy:
     runs-on: ubuntu-20.04

lib/sshkit/color.rb

@@ -44,8 +44,8 @@ def colorize(obj, color, mode=nil)
       return string unless COLOR_CODES.key?(color)
 
       result = mode == :bold ? "\e[1;" : "\e[0;"
-      result << COLOR_CODES.fetch(color).to_s
-      result << ";49m#{string}\e[0m"
+
+      "#{result}#{COLOR_CODES.fetch(color)};49m#{string}\e[0m"
     end
 
     # Returns `true` if the underlying output is a tty, or if the SSHKIT_COLOR

test/unit/backends/test_abstract.rb

@@ -112,7 +112,12 @@ def test_within_home
       end
 
       def test_background_logs_deprecation_warnings
-        deprecation_out = ''
+        deprecation_out =
+          if RUBY_VERSION < "2.3"
+            ''
+          else
+            +''
+          end
         SSHKit.config.deprecation_output = deprecation_out
 
         ExampleBackend.new do

test/unit/test_command.rb

@@ -211,7 +211,13 @@ def test_on_stderr
     end
 
     def test_deprecated_stdtream_accessors
-      deprecation_out = ''
+      deprecation_out =
+        if RUBY_VERSION < "2.3"
+          ''
+        else
+          +''
+        end
+
       SSHKit.config.deprecation_output = deprecation_out
 
       c = Command.new(:whoami)

test/unit/test_configuration.rb

@@ -11,7 +11,13 @@ def setup
     end
 
     def test_deprecation_output
-      output = ''
+      output =
+        if RUBY_VERSION < "2.3"
+          ''
+        else
+          +''
+        end
+
       SSHKit.config.deprecation_output = output
       SSHKit.config.deprecation_logger.log('Test')
       assert_equal "[Deprecated] Test\n", output.lines.first