Default to \n, retain consistent \r\n

When writing the Pipfile and Pipfile.lock make an effort to retain
their existing newlines if consistent.  Default to \n (LF) for new
files and files with inconsistent line endings.

tests/integration/test_project.py

@@ -1,4 +1,5 @@
 # -*- coding=utf-8 -*-
+import io
 import pytest
 import os
 from pipenv.project import Project
@@ -73,3 +74,36 @@ six = {{version = "*", index = "pypi"}}
             assert sorted(source.items()) == sorted(project.get_source(url=url).items())
             assert sorted(source.items()) == sorted(project.find_source(name).items())
             assert sorted(source.items()) == sorted(project.find_source(url).items())
+
+
+@pytest.mark.install
+@pytest.mark.project
+@pytest.mark.parametrize('newlines', [u'\n', u'\r\n'])
+@pytest.mark.parametrize('target', ['pipfile_path', 'lockfile_path'])
+def test_maintain_file_line_endings(PipenvInstance, pypi, newlines, target):
+    with PipenvInstance(pypi=pypi, chdir=True) as p:
+        path = getattr(p, target)
+
+        c = p.pipenv('install')
+        assert c.return_code == 0
+
+        with io.open(path) as f:
+            contents = f.read()
+
+        assert f.newlines == u'\n'
+
+        with io.open(path, 'w', newline=newlines) as f:
+            f.write(contents)
+
+        before = os.path.getmtime(path)
+
+        c = p.pipenv('install chardet')
+        assert c.return_code == 0
+
+        assert os.path.getmtime(path) != before
+
+        with io.open(path) as f:
+            f.read()
+            actual_newlines = f.newlines
+
+        assert actual_newlines == newlines