if no content-length, do not add the header

[CraigglesO]

Fixes errors thrown by Cloudflare. If content-length is 0, the request will error out.

[CLAassistant]

All committers have signed the CLA.

[dcr-stripe]

Thanks for the PR @CraigglesO and sorry for the delay in responding.

I think we'd actually want to do this on a per-method basis.

As per https://tools.ietf.org/html/rfc7230#section-3.3.2,

A user agent SHOULD NOT send a
Content-Length header field when the request message does not contain
a payload body and the method semantics do not anticipate such a
body.

We shouldn't be setting this header for verbs that don't anticipate a body.

However, for POST, we should keep sending Content-Length even if it is 0.

Any Content-Length field value greater than or equal to zero is
valid

The underlying undici library does this per-method as well: https://github.com/nodejs/undici/blob/cedc7d26f64aaa0571d1af6eaf82c519a1bcc6da/lib/client.js#L1403-L1407

[dcr-stripe]

I put together #1388 to do this on a per-method basis.

[dcr-stripe]

Closing this out now that #1388 has been merged - thanks again for the initial PR!