Fix parsing environment= with empty quoted strings

Pull #329 changed shlex to posix mode to fix quotes inside
quotes (#328).  A side effect of this change is that it
broke parsing empty quotes (#873).  This seems to be due
to a bug in shlex (http://bugs.python.org/issue21999).
Since no release version of Supervisor has shipped with
shlex in posix mode to support quotes inside quotes, we're
reverting it to fix support for empty quotes which has
shipped for many Supervisor versions.  Two unit tests
introduced in #329 pass without posix mode, so those tests
have been retained.  A unit test was also added for #873
in the previous commit.

Reopens #328
Partially reverts #329
Fixes #873
Closes #880

supervisor/datatypes.py

@@ -65,7 +65,7 @@ def dict_of_key_value_pairs(arg):
     """ parse KEY=val,KEY2=val2 into {'KEY':'val', 'KEY2':'val2'}
         Quotes can be used to allow commas in the value
     """
-    lexer = shlex.shlex(arg)
+    lexer = shlex.shlex(str(arg))
     lexer.wordchars += '/.+-():'
 
     tokens = list(lexer)

supervisor/tests/test_datatypes.py

@@ -162,11 +162,6 @@ def test_handles_newlines_inside_quotes(self):
         expected = {'foo': 'a\nb\nc'}
         self.assertEqual(actual, expected)
 
-    def test_handles_quotes_inside_quotes(self):
-        actual = datatypes.dict_of_key_value_pairs('foo="\'\\""')
-        expected = {'foo': '\'"'}
-        self.assertEqual(actual, expected)
-
     def test_handles_empty_inside_quotes(self):
         actual = datatypes.dict_of_key_value_pairs('foo=""')
         expected = {'foo': ''}