fix: http: send_request: Amend EXPECTING_BODY nil

Update the behavior of the HTTP library to handle cases where the
request body is nil for methods that expect a body
(e.g., POST, PUT, PATCH). Introduce an error message for such
cases and adjust the corresponding tests to align with the new
behavior.

References:
* https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/POST
* https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/PUT
* https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/PATCH

[sparschauer: extended commit description, wrapped a too long line,
 updated t/15-instance-reuse.t as well]
Co-authored-by: Sebastian Parschauer <Sebastian.Parschauer@ibm.com>

Author: chensunny

lib/resty/http.lua

@@ -706,7 +706,8 @@ function _M.send_request(self, params)
                 headers["Content-Length"] = length
 
             elseif body == nil and EXPECTING_BODY[str_upper(params.method)] then
-                headers["Content-Length"] = 0
+                return nil, "Request body is nil but " .. str_upper(params.method)
+                    .. " method expects a body. Use an empty string \"\" if you want to send an empty body."
 
             elseif body ~= nil then
                 headers["Content-Length"] = #tostring(body)

t/06-simpleinterface.t

@@ -269,15 +269,15 @@ nil closed
 lua tcp socket read timed out
 
 
-=== TEST 7: Content-Length is set on POST/PUT/PATCH requests when body is absent
+=== TEST 7: Content-Length is set on POST/PUT/PATCH requests when body is explicitly empty
 --- http_config eval: $::HttpConfig
 --- config
     location = /a {
         content_by_lua '
             for i, method in ipairs({ "POST", "PUT", "PATCH" }) do
               local http = require "resty.http"
               local httpc = http.new()
-              local res, err = httpc:request_uri("http://127.0.0.1:"..ngx.var.server_port.."/b", { method = method })
+              local res, err = httpc:request_uri("http://127.0.0.1:"..ngx.var.server_port.."/b", { method = method, body = "" })
 
               if not res then
                   ngx.log(ngx.ERR, err)
@@ -336,3 +336,36 @@ Content-Length: nil
 [error]
 [warn]
 
+
+=== TEST 9: Error when body is nil for POST/PUT/PATCH requests
+--- http_config eval: $::HttpConfig
+--- config
+    location = /a {
+        content_by_lua '
+            for i, method in ipairs({ "POST", "PUT", "PATCH" }) do
+              local http = require "resty.http"
+              local httpc = http.new()
+              local res, err = httpc:request_uri("http://127.0.0.1:"..ngx.var.server_port.."/b", { method = method })
+
+              if not res then
+                  ngx.say(method, ": ", err)
+              else
+                  ngx.say(method, ": success")
+              end
+            end
+        ';
+    }
+    location = /b {
+        content_by_lua '
+            ngx.say("Should not reach here")
+        ';
+    }
+--- request
+GET /a
+--- response_body
+POST: Request body is nil but POST method expects a body. Use an empty string "" if you want to send an empty body.
+PUT: Request body is nil but PUT method expects a body. Use an empty string "" if you want to send an empty body.
+PATCH: Request body is nil but PATCH method expects a body. Use an empty string "" if you want to send an empty body.
+--- no_error_log
+[error]
+[warn]

t/15-instance-reuse.t

@@ -101,13 +101,13 @@ location /a {
         }
 
         local res, err = httpc:request(params)
-        assert(res, "request should return positvely")
+        assert(res, "request should return positively")
 
         assert(not params.headers, "params table should not be modified")
 
         local res, err =
             httpc:request_uri("http://127.0.0.1:"..ngx.var.server_port, params)
-        assert(res, "request_uri should return positvely")
+        assert(res, "request_uri should return positively")
 
         assert(not params.headers, "params table should not be modified")
 
@@ -116,7 +116,7 @@ location /a {
             "connect should return positively")
 
         local pipeline_params = {
-            { path = "/b", method = "POST" },
+            { path = "/b", method = "POST", body = "" },
             { path = "/b", method = "HEAD" },
         }