Don't set Content-Length for GET requests without a BODY

The HTTP spec says (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

Author: jyn514

lib/http/request/writer.rb

@@ -47,7 +47,9 @@ def connect_through_proxy
       # Adds the headers to the header array for the given request body we are working
       # with
       def add_body_type_headers
-        return if @headers[Headers::CONTENT_LENGTH] || chunked?
+        return if @headers[Headers::CONTENT_LENGTH] || chunked? || (
+          @request_header[0].start_with?("GET ") && @body.source.nil?
+        )
 
         @request_header << "#{Headers::CONTENT_LENGTH}: #{@body.size}"
       end

spec/lib/http/request/writer_spec.rb

@@ -50,11 +50,10 @@
     context "when body is empty" do
       let(:body) { HTTP::Request::Body.new(nil) }
 
-      it "doesn't write anything to the socket and sets Content-Length" do
+      it "doesn't write anything to the socket and doesn't set Content-Length" do
         writer.stream
         expect(io.string).to eq [
-          "#{headerstart}\r\n",
-          "Content-Length: 0\r\n\r\n"
+          "#{headerstart}\r\n\r\n"
         ].join
       end
     end