Only seek in S3Boto3Storage._save if content is seekable

Also add tests to verify the behavior. This fixes issue #860.

storages/backends/s3boto3.py

@@ -463,7 +463,8 @@ def _save(self, name, content):
         name = self._normalize_name(cleaned_name)
         params = self._get_write_parameters(name, content)
 
-        content.seek(0, os.SEEK_SET)
+        if content.seekable():
+            content.seek(0, os.SEEK_SET)
         if (self.gzip and
                 params['ContentType'] in self.gzip_content_types and
                 'ContentEncoding' not in params):

tests/test_s3boto3.py

@@ -22,6 +22,18 @@ def setUp(self):
         self.storage._connections.connection = mock.MagicMock()
 
 
+class NonSeekableContentFile(ContentFile):
+
+    def open(self, mode=None):
+        return self
+
+    def seekable(self):
+        return False
+
+    def seek(self, pos, whence=0):
+        raise AttributeError()
+
+
 class S3Boto3StorageTests(S3Boto3TestCase):
 
     def test_clean_name(self):
@@ -121,6 +133,23 @@ def test_storage_save(self):
             }
         )
 
+    def test_storage_save_non_seekable(self):
+        """
+        Test saving a non-seekable file
+        """
+        name = 'test_storage_save.txt'
+        content = NonSeekableContentFile('new content')
+        self.storage.save(name, content)
+        self.storage.bucket.Object.assert_called_once_with(name)
+
+        obj = self.storage.bucket.Object.return_value
+        obj.upload_fileobj.assert_called_with(
+            content,
+            ExtraArgs={
+                'ContentType': 'text/plain',
+            }
+        )
+
     def test_storage_save_with_default_acl(self):
         """
         Test saving a file with user defined ACL.
@@ -194,6 +223,22 @@ def test_storage_save_gzipped(self):
             }
         )
 
+    def test_storage_save_gzipped_non_seekable(self):
+        """
+        Test saving a gzipped file
+        """
+        name = 'test_storage_save.gz'
+        content = NonSeekableContentFile("I am gzip'd")
+        self.storage.save(name, content)
+        obj = self.storage.bucket.Object.return_value
+        obj.upload_fileobj.assert_called_with(
+            content,
+            ExtraArgs={
+                'ContentType': 'application/octet-stream',
+                'ContentEncoding': 'gzip',
+            }
+        )
+
     def test_storage_save_gzip(self):
         """
         Test saving a file with gzip enabled.