Conditionally defining #blank? on String/NilClass

Signed-off-by: Chris Eppstein <chris@eppsteins.net>

lib/compass/core_ext.rb

@@ -1,12 +1,14 @@
 class String
-
-  # see if string has any content
-  def blank?; self.length.zero?; end
-  
+  unless method_defined?(:blank?)
+    # see if string has any content
+    def blank?; self.length.zero?; end
+  end
 end
 
 class NilClass
-  def blank?
-    true
+  unless method_defined?(:blank?)
+    def blank?
+      true
+    end
   end
 end