Port parse_mime_headers() to Python 3

Port swift.common.utils.parse_mime_headers() to Python 3:

* On Python 3, tries to decode headers from UTF-8. If an header was
  was not encoded to UTF-8, decode the header from Latin1.
* Update the parse_mime_headers() tests: on Python 3, HTTP header
  values are Unicode strings.

This change is a follow-up of the change
Ia5ee2ead67e36e8c6416183667f64ae255887736.

Change-Id: I042dd13e9eb0e9844ccd832d538cdac84359ed42

swift/common/utils.py

@@ -3421,11 +3421,21 @@ def parse_mime_headers(doc_file):
     headers = []
     while True:
         line = doc_file.readline()
+        done = line in (b'\r\n', b'\n', b'')
+        if six.PY3:
+            try:
+                line = line.decode('utf-8')
+            except UnicodeDecodeError:
+                line = line.decode('latin1')
         headers.append(line)
-        if line in (b'\r\n', b'\n', b''):
+        if done:
             break
-    header_string = b''.join(headers)
-    return HeaderKeyDict(email.parser.Parser().parsestr(header_string))
+    if six.PY3:
+        header_string = ''.join(headers)
+    else:
+        header_string = b''.join(headers)
+    headers = email.parser.Parser().parsestr(header_string)
+    return HeaderKeyDict(headers)
 
 
 def mime_to_document_iters(input_file, boundary, read_chunk_size=4096):

test/unit/common/test_utils.py

@@ -5006,6 +5006,10 @@ Utf-8: \xd0\xba\xd0\xbe\xd0\xbd\xd1\x82\xd0\xb5\xd0\xb9\xd0\xbd\xd0\xb5\xd1\x80
 This is the body
 """)
         headers = utils.parse_mime_headers(doc_file)
+        utf8 = u'\u043a\u043e\u043d\u0442\u0435\u0439\u043d\u0435\u0440'
+        if six.PY2:
+            utf8 = utf8.encode('utf-8')
+
         expected_headers = {
             'Content-Disposition': 'form-data; name="file_size"',
             'Foo': "Bar",
@@ -5015,8 +5019,7 @@ This is the body
             'Connexion': "=?iso8859-1?q?r=E9initialis=E9e_par_l=27homologue?=",
             'Status': "=?utf-8?b?5byA5aeL6YCa6L+H5a+56LGh5aSN5Yi2?=",
             'Latin-1': "Resincronizaci\xf3n realizada con \xe9xito",
-            'Utf-8': ("\xd0\xba\xd0\xbe\xd0\xbd\xd1\x82\xd0\xb5\xd0\xb9\xd0"
-                      "\xbd\xd0\xb5\xd1\x80")
+            'Utf-8': utf8,
         }
         self.assertEqual(expected_headers, headers)
         self.assertEqual(b"This is the body\n", doc_file.read())