Only respect :using specs when downloading HEAD.

When a formula specifies both an `url` and a `head`, and the latter
specified a `:using` spec, brew would try to fetch the `url` using the
specified download strategy.  With this change, brew respects `:using`
specs only when determining the download strategy for `head`.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/formula.rb

@@ -134,7 +134,7 @@ class Formula
   
   # reimplement if we don't autodetect the download strategy you require
   def download_strategy
-    if @specs
+    if @specs and @url == @head
       vcs = @specs.delete :using
       if vcs != nil
         # If a class is passed, assume it is a download strategy