[ruby] fix FrozenError: can't modify frozen String exception

rb/lib/selenium/webdriver/firefox/binary.rb

@@ -53,8 +53,8 @@ def path
             @path = Platform.cygwin_path(@path, windows: true) if Platform.cygwin?
 
             unless File.file?(@path.to_s)
-              error = "Could not find Firefox binary (os=#{Platform.os}). "
-              error << "Make sure Firefox is installed or set the path manually with #{self}.path="
+              error = "Could not find Firefox binary (os=#{Platform.os}). " \
+                      "Make sure Firefox is installed or set the path manually with #{self}.path="
               raise Error::WebDriverError, error
             end
 

rb/spec/unit/selenium/webdriver/firefox/binary_spec.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+# Licensed to the Software Freedom Conservancy (SFC) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The SFC licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+require File.expand_path('../spec_helper', __dir__)
+
+module Selenium
+  module WebDriver
+    module Firefox
+      describe Binary do
+        describe ".path" do
+          context "when could't find binary file" do
+            it 'raises Selenium::WebDriver::Error::WebDriverError' do
+              expect(File).to receive(:file?).and_return(false)
+
+              expect { Binary.path }.to raise_error(Selenium::WebDriver::Error::WebDriverError)
+            end
+          end
+        end
+      end
+    end # Firefox
+  end # WebDriver
+end # Selenium