Rename internal qbxml instance used for serialization

Originally the "parser" was used for both parsing and serializing
qbxml, but these are now separate so it makes more sense to just
name this as such.

Author: mwojo

lib/qbwc.rb

@@ -145,7 +145,7 @@ def parser
       @@parser[min_version] ||= Qbxml.new(api, min_version)
     end
 
-    def write_parser(session = nil)
+    def serializer(session = nil)
       ver = serialization_version.is_a?(Proc) ? serialization_version.call(session) : serialization_version
       ver ||= min_version
       @@parser ||= {}

lib/qbwc/request.rb

@@ -9,7 +9,7 @@ def initialize(request, session)
     case
     when request.is_a?(Hash)
       request = self.class.wrap_request(request)
-      @request = QBWC.write_parser(session).to_qbxml(request, {:validate => true})
+      @request = QBWC.serializer(session).to_qbxml(request, {:validate => true})
     when request.is_a?(String)
       @request = request
     else
@@ -18,7 +18,7 @@ def initialize(request, session)
   end
 
   def to_qbxml
-    QBWC.write_parser.to_qbxml(request)
+    QBWC.serializer.to_qbxml(request)
   end
 
   def to_hash