Validate that proc passed to add_before_exec...

Previously, you could call RestClient.add_before_execution_proc() with
no arguments or block, and it would add `nil` to the list of procs. This
would cause a nil NoMethodError down the line. Instead, immediately
raise an ArgumentError.

lib/restclient.rb

@@ -166,6 +166,7 @@ module RestClient
   # Add a Proc to be called before each request in executed.
   # The proc parameters will be the http request and the request params.
   def self.add_before_execution_proc &proc
+    raise ArgumentError.new('block is required') unless proc
     @@before_execution_procs << proc
   end