Fix newline handling in MultipartStreamTest (#461)
Fix newline handling in MultipartStreamTest (#461)

* Do not depend on the newline of the file in MultipartStreamTest

Resolves guzzle/psr7#460

* Add MultipartStreamTest::testSerializesFilesWithMixedNewlines()

This test ensures that newlines are serialized as-is and that the tests do not
normalize newlines in assertions.
guzzle/psr7