Remove X-Records header parsing

[bhelx]

We are removing the X-Records header in api version 2.6 on paginating GET requests
and they only come back on HEAD requests. Now calling Resource#count or Pager#count
will always explicitly call the server with a HEAD request. This should maybe be considered a breaking change:

To get a count, you'll need to explicitly call count which will call the server on each call.

params = {
  begin_time: Date.today.prev_month,
  order: :asc,
  sort: :created_at
}

count = Recurly::Transaction.where(params).count
puts count

[lmnleaf]

+1

[bhelx]

Okay, squashed