Fix for Process: ensure chdir is a string

spec/std/process_spec.cr

@@ -180,7 +180,7 @@ describe Process do
       $?.exit_code.should eq(0)
     end
 
-    it "sets working directory" do
+    it "sets working directory with string" do
       parent = File.dirname(Dir.current)
       command = {% if flag?(:win32) %}
                   "cmd.exe /c echo %cd%"
@@ -193,6 +193,19 @@ describe Process do
       value.should eq "#{parent}#{newline}"
     end
 
+    it "sets working directory with path" do
+      parent = Path.new File.dirname(Dir.current)
+      command = {% if flag?(:win32) %}
+                  "cmd.exe /c echo %cd%"
+                {% else %}
+                  "pwd"
+                {% end %}
+      value = Process.run(command, shell: true, chdir: parent, output: Process::Redirect::Pipe) do |proc|
+        proc.output.gets_to_end
+      end
+      value.should eq "#{parent}#{newline}"
+    end
+
     pending_win32 "disallows passing arguments to nowhere" do
       expect_raises ArgumentError, /args.+@/ do
         Process.run("foo bar", {"baz"}, shell: true)

src/process.cr

@@ -243,7 +243,7 @@ class Process
     fork_output = stdio_to_fd(output, for: STDOUT)
     fork_error = stdio_to_fd(error, for: STDERR)
 
-    pid = Crystal::System::Process.spawn(command_args, env, clear_env, fork_input, fork_output, fork_error, chdir)
+    pid = Crystal::System::Process.spawn(command_args, env, clear_env, fork_input, fork_output, fork_error, chdir.try &.to_s)
     @process_info = Crystal::System::Process.new(pid)
 
     fork_input.close unless fork_input.in?(input, STDIN)