Add Assoc For Hashes as a ruby til.

ruby/assoc-for-hashes.md

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+# Assoc For Hashes
+
+Ruby's Hash class comes with a method, `assoc`, which is good for grabbing
+both the key and value from a hash. If the given key matches it returns a
+two element array with the key and value.
+
+```ruby
+> stuff = {a: 1, b: 2, c: 3}
+=> {a: 1, b: 2, c: 3}
+> stuff.assoc(:c)
+=> [:c, 3]
+```
+
+If a key is used that doesn't exist in the hash, then it simply returns nil.
+
+```ruby
+> {}.assoc(:c)
+=> nil
+```