If the download is a .jar, don't unzip it

Also remove special casing for two existing jar formula.

Library/Homebrew/download_strategy.rb

@@ -75,9 +75,14 @@ class CurlDownloadStrategy <AbstractDownloadStrategy
 
   def stage
     # magic numbers stolen from /usr/share/file/magic/
-    File.open(@dl) do |f|
+    if @dl.extname == '.jar'
+      magic_bytes = nil
+    else
       # get the first four bytes
-      case f.read(4)
+      File.open(@dl) { |f| magic_bytes = f.read(4) }
+    end
+
+    case magic_bytes
     when /^PK\003\004/ # .zip archive
       quiet_safe_system '/usr/bin/unzip', {:quiet_flag => '-qq'}, @dl
       chdir
@@ -91,10 +96,11 @@ class CurlDownloadStrategy <AbstractDownloadStrategy
       # So I guess we should cp, but we mv, for this historic reason
       # HOWEVER if this breaks some expectation you had we *will* change the
       # behaviour, just open an issue at github
+      # We also do this for jar files, as they are in fact zip files, but
+      # we don't want to unzip them
       FileUtils.mv @dl, File.basename(@url)
     end
   end
-  end
 
 private
   def chdir