Fix issue #22

[matbech]

Remove the subject header otherwise it will be duplicated.

[mwillbanks]

@matbech can you please add tests for this?

[matbech]

The Subject header is added by the php mail function on Windows. If an extra Subject header is present in the header array the result is a message with two Subject headers.

The request for an automated test is not reasonable because:
1.There is already enough evidence of the bug:

• Manual test (reason for this PR) and detailed description in Subject header is duplicated #22
• php source where the "Subject" header is added:
  https://github.com/php/php-src/blob/0437aa2abf1eb494e4555d7d4dfbdbf566a59bdc/win32/sendmail.c#L595
  https://github.com/php/php-src/blob/0437aa2abf1eb494e4555d7d4dfbdbf566a59bdc/win32/sendmail.c#L702

2.Writing an STMP server to intercept the mail communication from Window's php:mail is unreasonable, considering the evidence already presented.

[glensc]

@matbech can you just make test that if OS is windows the header is removed? not sure if the isWindows result can be mocked.

as for the proof, maybe it's behavior change in php 7, and behaving differently in php < 7

[matbech]

I'm sorry but I have a hard time to understand your sentence. This has been tested on Windows with PHP 7. There is no good reason to leave this bug open for more than 6 months now.

[glensc]

Did you test all other supported versions too?, PHP < 7

I'm trying to help, i don't care for one bit for windows support personally!
as it's hard to understand how the original code landed, it probably behaved differently back then, or different windows, the other option would be the code was committed without any testing, i just find that hard to believe!

[weierophinney]

We've asked for tests, and blocked this for lack of tests; it's not a question of whether or not we want the fix, but wanting to ensure it stays fixed.

You can test this by mocking the result of what isWindowsOs() returns (potentially by extending the transport to hardcode the value), and checking the message generated to see if the header was removed.

If/when you are able to do that, please open a new pull request; until then, I'm closing this for now.