fix: curl async http client empty-valued header

[curme]

Behavior

I found that some of the requests I sent by tornado would lose part of headers, exactly the empty-valued ones.

For example, when posting a request with headers {'Content-Type': "application/json", 'Authorization': ""}, only header Content-Type will be sent out actually, with the header Authorization dropped because it's empty.

For some requests, header with blank content is indispensable. In my case, I met an odd server must make sure there is an Authorization in headers even it is blank.

Reason

The behavior caused by the way curl-http-client constructing headers, which converting all of headers in "%s: %s" format. The headers having no value, such as 'xxxx: ', would be removed by cURL.

To keep those headers in request, the colon should be replaced with semicolon, such as 'xxxx;'. libcurl examples

[bdarnell]

This functionality needs a test to demonstrate that it's working correctly (and curl is translating the semicolon into a colon for the server's consumption). A test in httpclient_test would also verify that simple_httpclient is handling this edge case correctly.

It would be nice to link to the docs for CURLOPT_HTTPHEADER because the syntax used by this feature is quite surprising.

[bdarnell]

This should be != (or just if v), not is not. The use of is here is relying on interning of small strings which I believe is an undocumented implementation detail.