check for content-length header before setting it in nextTick

extending https://github.com/mikeal/request/pull/793 so it does not override a custom content-length header.

Especially relevant when passing in a stream since `_form.getLengthSync` [doesn't calculate](https://github.com/felixge/node-form-data/blob/fdc52f3fa32198da44644bb158e6f5764feb3767/test/integration/test-errors.js#L61-L62) the length of the stream in form-data, making this statement: https://github.com/mikeal/request/issues/345#issuecomment-13818530 not technically correct.