Add Check Specific Arguments To Received Method as an RSpec til

rspec/check-specific-arguments-to-received-method.md

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+# Check Specific Arguments To Received Method
+
+Let's say we have a method receiving a big hash of arguments. A hash like this:
+
+```ruby
+{
+  name: 'Taco Tray',
+  product_id: 'taco123',
+  price: 4500,
+  description: 'A big tray of tacos',
+  discounts: {
+    coupon: 'DISCOUNT_TACOS'
+  }
+}
+```
+
+In an RSpec test we want to check one of those hash values in a certain
+scenario. It can be tedious to type out and check the entire hash. Instead, we
+want the test to surgically check just one part of the hash.
+
+We can do this with RSpec's dynamic matcher syntax. The [`hash_including`
+argument
+matcher](https://rspec.info/documentation/3.4/rspec-mocks/RSpec/Mocks/ArgumentMatchers.html#hash_including-instance_method)
+can be nested within the `#with` part of `expect().to receive().with()`.
+
+```ruby
+expect(TacoTruck)
+  .to receive(:take_order)
+  .with(
+    hash_including(
+      discounts: { coupon: 'DISCOUNT_TACOS'}
+    )
+  )
+```
+
+This will assert about the `discounts` portion of the hash that `#take_order`
+gets called with. The rest of the hash will be ignored.
+
+Without `hash_including`, the `with` call would result in a failure when trying
+to match against the entire hash.