livecheck: Include Content-Length header for POST

Some servers will return an error response if a `Content-Length`
header isn't included in a `POST` request, so this adds it to the
`post_args` array when `post_form` or `post_json` are used.

Library/Homebrew/livecheck/strategy.rb

@@ -178,7 +178,7 @@ module Homebrew
         ).returns(T::Array[String])
       }
       def self.post_args(post_form: nil, post_json: nil)
-        if post_form.present?
+        args = if post_form.present?
           require "uri"
           ["--data", URI.encode_www_form(post_form)]
         elsif post_json.present?
@@ -187,6 +187,12 @@ module Homebrew
         else
           []
         end
+
+        if (content_length = args[1]&.length)
+          args << "--header" << "Content-Length: #{content_length}"
+        end
+
+        args
       end
 
       # Collects HTTP response headers, starting with the provided URL.

Library/Homebrew/test/livecheck/strategy_spec.rb

@@ -144,16 +144,22 @@ RSpec.describe Homebrew::Livecheck::Strategy do
   end
 
   describe "::post_args" do
+    let(:form_string_content_length) { "Content-Length: #{form_string.length}" }
+    let(:json_string_content_length) { "Content-Length: #{json_string.length}" }
+
     it "returns an array including `--data` and an encoded form data string" do
-      expect(strategy.post_args(post_form: post_hash)).to eq(["--data", form_string])
+      expect(strategy.post_args(post_form: post_hash))
+        .to eq(["--data", form_string, "--header", form_string_content_length])
 
       # If both `post_form` and `post_json` are present, only `post_form` will
       # be used.
-      expect(strategy.post_args(post_form: post_hash, post_json: post_hash)).to eq(["--data", form_string])
+      expect(strategy.post_args(post_form: post_hash, post_json: post_hash))
+        .to eq(["--data", form_string, "--header", form_string_content_length])
     end
 
     it "returns an array including `--json` and a JSON string" do
-      expect(strategy.post_args(post_json: post_hash)).to eq(["--json", json_string])
+      expect(strategy.post_args(post_json: post_hash))
+        .to eq(["--json", json_string, "--header", json_string_content_length])
     end
 
     it "returns an empty array if `post_form` value is blank" do