Quote arguments with whitespace when printing command

Currently, printing baked commands doesn't represent the way arguments
are actually passed down to the program when values contain spaces.

The solution is to take such values in single quotes.

This is primarily to be able to copy printed commands
and run them in a shell.

BEFORE:

```python
>>> from sh import ls
>>> print(ls.bake('How I Met Your Mother'))
/bin/ls How I Met Your Mother
```

AFTER:

```python
>>> print(ls.bake('How I Met Your Mother'))
/bin/ls 'How I Met Your Mother'
```

Author: peterdemin

sh.py

@@ -1391,10 +1391,10 @@ def bake(self, *args, **kwargs):
         return fn
 
     def __str__(self):
-        baked_args = " ".join(self._partial_baked_args)
-        if baked_args:
-            baked_args = " " + baked_args
-        return self._path + baked_args
+        if not self._partial_baked_args:
+            return self._path
+        baked_args = " ".join(shlex_quote(arg) for arg in self._partial_baked_args)
+        return f"{self._path} {baked_args}"
 
     def __eq__(self, other):
         return str(self) == str(other)

tests/sh_test.py

@@ -2686,6 +2686,18 @@ def test_baked_command_can_be_printed(self):
         ll = ls.bake("-l")
         self.assertTrue(str(ll).endswith("/ls -l"))
 
+    def test_baked_command_can_be_printed_with_whitespace_args(self):
+        from sh import ls
+
+        ls_himym = ls.bake("How I Met Your Mother")
+        self.assertTrue(str(ls_himym).endswith("/ls 'How I Met Your Mother'"))
+        ls_himym = ls.bake("How I 'Met' Your Mother")
+        self.assertTrue(
+            str(ls_himym).endswith("""/ls 'How I '"'"'Met'"'"' Your Mother'""")
+        )
+        ls_himym = ls.bake('How I "Met" Your Mother')
+        self.assertTrue(str(ls_himym).endswith("""/ls 'How I "Met" Your Mother'"""))
+
     # https://github.com/amoffat/sh/issues/185
     def test_done_callback(self):
         import time