Fixing regression from block delimiter enhancement

When I tried to bump Shopify to use the latest version of Liquid, a unit test failed. I took the test and moved it into the Liquid test suite and it failed there as well.

The block_delimiter enhancement I did in #384 was causing the failure. I reverted it to only create the delimiter on first encoutering it, but I memoized it so that we don't recreate it on every run through.

@Shopify/liquid