Start documenting dependency contracts with unit tests

Implement some unit tests for 'email' package so we know precisely how this
package behaves.

test/test_contracts.py

@@ -0,0 +1,62 @@
+#
+#	gpg-mailgate
+#
+#	This file is part of the gpg-mailgate source code.
+#
+#	gpg-mailgate is free software: you can redistribute it and/or modify
+#	it under the terms of the GNU General Public License as published by
+#	the Free Software Foundation, either version 3 of the License, or
+#	(at your option) any later version.
+#
+#	gpg-mailgate source code is distributed in the hope that it will be useful,
+#	but WITHOUT ANY WARRANTY; without even the implied warranty of
+#	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+#	GNU General Public License for more details.
+#
+#	You should have received a copy of the GNU General Public License
+#	along with gpg-mailgate source code. If not, see <http://www.gnu.org/licenses/>.
+#
+
+"""Unit-tests as contracts for external dependencies.
+
+Unit tests defined here are our contracts for the dependencies used by Lacre.
+Since not all software is documented thoroughly, they are also a form of
+documentation.
+"""
+
+import email
+import unittest
+
+class EmailParsingTest(unittest.TestCase):
+	"""This test serves as a package contract and documentation of its behaviour."""
+
+	def test_message_from_bytes_produces_message_with_str_headers(self):
+		rawmsg = b"From: alice@lacre.io\r\n" \
+		  + b"To: bob@lacre.io\r\n" \
+		  + b"Subject: Test message\r\n" \
+		  + b"\r\n" \
+		  + b"Test message from Alice to Bob.\r\n"
+
+		parsed = email.message_from_bytes(rawmsg)
+
+		self.assertEqual(parsed["From"], "alice@lacre.io")
+		self.assertEqual(parsed["To"], "bob@lacre.io")
+		self.assertEqual(parsed["Subject"], "Test message")
+		self.assertEqual(parsed.get_payload(), "Test message from Alice to Bob.\r\n")
+
+	def test_message_from_string_produces_message_with_str_headers(self):
+		rawmsg = "From: alice@lacre.io\r\n" \
+		  + "To: bob@lacre.io\r\n" \
+		  + "Subject: Test message\r\n" \
+		  + "\r\n" \
+		  + "Test message from Alice to Bob.\r\n"
+
+		parsed = email.message_from_string(rawmsg)
+
+		self.assertEqual(parsed["From"], "alice@lacre.io")
+		self.assertEqual(parsed["To"], "bob@lacre.io")
+		self.assertEqual(parsed["Subject"], "Test message")
+		self.assertEqual(parsed.get_payload(), "Test message from Alice to Bob.\r\n")
+
+if __name__ == '__main__':
+	unittest.main()