Remove Content-Type Header when request with empty body POST method

[jay-choe]

Hi.

I have looked this issue and got an idea to deal with.

In current code, convert null body to not null and it write to outputstream.

In that process, when writing empty string in outputstream, unwanted Content-Type Header is added which is ''application/x-www-form-urlencoded".

So, I rearrange null body code to avoid unwanted header.

With this, Content-Length header is not set, so I added. But, this header is restricted header so 'sun.net.http.allowRestrictedHeaders' system property should be true to add content-length header.

And this property should be set before client class loaded. I guess there is no suitable way to inject restricted header to connection.

Feel Free to comment this idea. thanks.

[kdavisk6]

This look good. Could you please add some information to the README to indicate how to take advantage of this change, helping folks understand the System Property requirement?

[jay-choe]

@kdavisk6 sure, I added it.

[velo]

@jay-choe @kdavisk6 FWIW, I have rebased this PR after the formatting changes I did on master

[zoomout]

@velo
Please revert this change. Setting "sun.net.http.allowRestrictedHeaders" is breaking security.
restrictedHeaders include e.g. Host, enabling CSRF attack.

private static final String[] restrictedHeaders = {
    /* Restricted by XMLHttpRequest2 */
    //"Accept-Charset",
    //"Accept-Encoding",
    "Access-Control-Request-Headers",
    "Access-Control-Request-Method",
    "Connection", /* close is allowed */
    "Content-Length",
    //"Cookie",
    //"Cookie2",
    "Content-Transfer-Encoding",
    //"Date",
    //"Expect",
    "Host",
    "Keep-Alive",
    "Origin",
    // "Referer",
    // "TE",
    "Trailer",
    "Transfer-Encoding",
    "Upgrade",
    //"User-Agent",
    "Via"
};

[velo]

Hello, good morning, how are you today?

Raise a PR with test case.

Thank you.