make hmac stuff a bit more flexible

lib/fog/hmac.rb

@@ -2,17 +2,33 @@ module Fog
   class HMAC
 
     def initialize(type, key)
-      @digest = case type
-      when 'sha1'
-        OpenSSL::Digest::Digest.new('sha1')
-      when 'sha256'
-        OpenSSL::Digest::Digest.new('sha256')
-      end
       @key = key
+      case type
+      when 'sha1'
+        setup_sha1
+      when 'sha256'
+        setup_sha256
+      end
     end
 
     def sign(data)
-      OpenSSL::HMAC.digest(@digest, @key, data)
+      @signer.call(data)
+    end
+
+    private
+
+    def setup_sha1
+      @digest = OpenSSL::Digest::Digest.new('sha1')
+      @signer = lambda do |data|
+        OpenSSL::HMAC.digest(@digest, @key, data)
+      end
+    end
+
+    def setup_sha256
+      @digest = OpenSSL::Digest::Digest.new('sha256')
+      @signer = lambda do |data|
+        OpenSSL::HMAC.digest(@digest, @key, data)
+      end
     end
 
   end