Fig bug in split_buffer where input was being discarded

Also, write some tests for it.

tests/split_buffer_test.py

@@ -0,0 +1,37 @@
+from __future__ import unicode_literals
+from __future__ import absolute_import
+from fig.cli.utils import split_buffer
+from . import unittest
+
+class SplitBufferTest(unittest.TestCase):
+    def test_single_line_chunks(self):
+        def reader():
+            yield "abc\n"
+            yield "def\n"
+            yield "ghi\n"
+
+        self.assertEqual(list(split_buffer(reader(), '\n')), ["abc\n", "def\n", "ghi\n"])
+
+    def test_no_end_separator(self):
+        def reader():
+            yield "abc\n"
+            yield "def\n"
+            yield "ghi"
+
+        self.assertEqual(list(split_buffer(reader(), '\n')), ["abc\n", "def\n", "ghi"])
+
+    def test_multiple_line_chunk(self):
+        def reader():
+            yield "abc\ndef\nghi"
+
+        self.assertEqual(list(split_buffer(reader(), '\n')), ["abc\n", "def\n", "ghi"])
+
+    def test_chunked_line(self):
+        def reader():
+            yield "a"
+            yield "b"
+            yield "c"
+            yield "\n"
+            yield "d"
+
+        self.assertEqual(list(split_buffer(reader(), '\n')), ["abc\n", "d"])