Fix total_pages method fail on active_record model after calling merge with unscoped limit

[allomov]

It has been observed that using active record .merge can result in having .limit_value as nil. This can cause .total_entries and .total_pages to fail with the error comparison of Integer with nil failed on the following line:

will_paginate/lib/will_paginate/active_record.rb

Line 70 in 573df2d

if loaded? and size < limit_value and (current_page == 1 or size > 0)

Here is an example that demonstrates how you can get a nil value in the limit_value field:

User.limit(10).limit_value
# => 10
User.joins(:role).limit(10).merge(Role.all.unscope(:limit)).limit_value
# => nil

There are two places that require updates:

• The total_entries method in active_record.rb fails when limit_value is nil and compared to the size https://github.com/allomov/will_paginate/blob/53e08fe99b4bbfa67213ac28734871bacb59e343/lib/will_paginate/active_record.rb#L70
• In the per_page method, limit_value can cause issues while computing .total_pages https://github.com/allomov/will_paginate/blob/53e08fe99b4bbfa67213ac28734871bacb59e343/lib/will_paginate/active_record.rb#L27