ActiveRecordWrapper limit should default to 100, as doc'd

Too hard to figure out an automated test for this right now, but I did manually confirm

Author: jrochkind

lib/oai/provider.rb

@@ -146,8 +146,8 @@
 # * `timestamp_field` - Specifies the model field to use as the update
 #   filter.  Defaults to `updated_at`.
 # * `limit` -           Maximum number of records to return in each page/set.
-#   Defaults to 100.
-#   The wrapper will paginate the result via resumption tokens.
+#   Defaults to 100, set to `nil` for all records in one page. Otherwise
+#   the wrapper will paginate the result via resumption tokens.
 #   _Caution:  specifying too large a limit will adversely affect performance._
 #
 # Mapping from a ActiveRecord object to a specific metadata format follows

lib/oai/provider/model/activerecord_wrapper.rb

@@ -15,7 +15,7 @@ class ActiveRecordWrapper < Model
     def initialize(model, options={})
       @model = model
       @timestamp_field = options.delete(:timestamp_field) || 'updated_at'
-      @limit = options.delete(:limit)
+      @limit = options.delete(:limit) || 100
 
       unless options.empty?
         raise ArgumentError.new(