Cask handle already downloaded file

The current logic for curl_download tries to restart any existing
partial download, but this fails for some servers when the file is
already fully downloaded, returning a status code 416. This updates the
function to check for the file being already downloaded and if so bails
early.

The following command used to fail when run more than once but now works
as expected:
```
brew cask info https://raw.githubusercontent.com/homebrew/homebrew-cask/master/Casks/docker.rb
```

Library/Homebrew/test/cask/cmd/info_spec.rb

@@ -136,4 +136,32 @@ describe Cask::Cmd::Info, :cask do
       Caffeine.app (App)
     EOS
   end
+
+  it "can run be run with a url twice" do
+    expect {
+      described_class.run("https://raw.githubusercontent.com/Homebrew/homebrew-cask" \
+                          "/d0b2c58652ae5eff20a7a4ac93292a08b250912b/Casks/docker.rb")
+      described_class.run("https://raw.githubusercontent.com/Homebrew/homebrew-cask" \
+                          "/d0b2c58652ae5eff20a7a4ac93292a08b250912b/Casks/docker.rb")
+    }.to output(<<~EOS).to_stdout
+      ==> Downloading https://raw.githubusercontent.com/Homebrew/homebrew-cask/d0b2c58652ae5eff20a7a4ac93292a08b250912b/Casks/docker.rb.
+      docker: 2.0.0.2-ce-mac81,30215 (auto_updates)
+      https://www.docker.com/community-edition
+      Not installed
+      ==> Names
+      Docker Community Edition
+      Docker CE
+      ==> Artifacts
+      Docker.app (App)
+      ==> Downloading https://raw.githubusercontent.com/Homebrew/homebrew-cask/d0b2c58652ae5eff20a7a4ac93292a08b250912b/Casks/docker.rb.
+      docker: 2.0.0.2-ce-mac81,30215 (auto_updates)
+      https://www.docker.com/community-edition
+      Not installed
+      ==> Names
+      Docker Community Edition
+      Docker CE
+      ==> Artifacts
+      Docker.app (App)
+    EOS
+  end
 end

Library/Homebrew/utils/curl.rb

@@ -51,10 +51,20 @@ def curl_download(*args, to: nil, **options)
   destination = Pathname(to)
   destination.dirname.mkpath
 
-  continue_at = if destination.exist? &&
-                   curl_output("--location", "--range", "0-1",
-                               "--write-out", "%{http_code}",
-                               "--output", "/dev/null", *args, **options).stdout.to_i == 206 # Partial Content
+  range_stdout = curl_output("--location", "--range", "0-1",
+                             "--dump-header", "-",
+                             "--write-out", "%{http_code}",
+                             "--output", "/dev/null", *args, **options).stdout
+  headers, _, http_status = range_stdout.partition("\r\n\r\n")
+
+  supports_partial_download = http_status.to_i == 206 # Partial Content
+  if supports_partial_download &&
+     destination.exist? &&
+     destination.size == %r{^.*Content-Range: bytes \d+-\d+/(\d+)\r\n.*$}m.match(headers)[1].to_i
+    return # We've already downloaded all the bytes
+  end
+
+  continue_at = if destination.exist? && supports_partial_download
     "-"
   else
     0