(#8716) Dup string if frozen for line endings

Prior to this commit, if a user passed in a script that was frozen,
the shell provisioner would fail to modify the script to replace line
endings for windows because the string was immutable. This commit fixes
that by dup'ing the string so that it can have its line endings replaced

plugins/provisioners/shell/provisioner.rb

@@ -265,7 +265,7 @@ module VagrantPlugins
         # or we're running on Windows.
         if !config.binary && @machine.config.vm.communicator != :winrm
           begin
-            script.gsub!(/\r\n?$/, "\n")
+            script = script.gsub(/\r\n?$/, "\n")
           rescue ArgumentError
             script = script.force_encoding("ASCII-8BIT").gsub(/\r\n?$/, "\n")
           end

test/unit/plugins/provisioners/shell/provisioner_test.rb

@@ -43,6 +43,37 @@ describe "Vagrant::Shell::Provisioner" do
     end
   end
 
+  context "with a script that was set to freeze the string" do
+    TEST_CONSTANT_VARIABLE = <<-TEST_CONSTANT_VARIABLE.freeze
+      echo test
+    TEST_CONSTANT_VARIABLE
+
+    let(:script) { TEST_CONSTANT_VARIABLE }
+    let(:config) {
+      double(
+        :config,
+        :args        => "doesn't matter",
+        :env         => {},
+        :upload_path => "arbitrary",
+        :remote?     => false,
+        :path        => nil,
+        :inline      => script,
+        :binary      => false,
+      )
+    }
+
+    it "does not raise an exception" do
+      vsp = VagrantPlugins::Shell::Provisioner.new(machine, config)
+
+      RSpec::Expectations.configuration.on_potential_false_positives = :nothing
+      # This test should be fine, since we are specifically looking for the
+      # string 'freeze' when RuntimeError is raised
+      expect {
+        vsp.provision
+      }.not_to raise_error(RuntimeError)
+    end
+  end
+
   context "with remote script" do
 
     context "that does not have matching sha1 checksum" do