BUG: merge_asof not handling allow_exact_matches and tolerance on first entry
closes #13695

@chrisaycock if you have more examples happy to add.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13698?src=pr) is **84.38%**

> Merging [#13698](https://codecov.io/gh/pydata/pandas/pull/13698?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13698   diff @@
==========================================
  Files           142        142          
  Lines         51223      51223          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43224      43224          
  Misses         7999       7999          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [ada6bf3...c46dcfa](https://codecov.io/gh/pydata/pandas/compare/ada6bf350f7fd4daaf2b80188ca165fb9543a252...c46dcfadceb033167f864c3c8925aee39220cdc4?src=pr)
