Update md files

Author: Braktar

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 * [#2161](https://github.com/ruby-grape/grape/pull/2157): Handle EOFError from Rack when given an empty multipart body - [@bschmeck](https://github.com/bschmeck).
 * [#2162](https://github.com/ruby-grape/grape/pull/2162): Corrected a hash modification while iterating issue - [@Jack12816](https://github.com/Jack12816).
+* [#2164](https://github.com/ruby-grape/grape/pull/2164): coerce_with is called for params with nil value - [@braktar](https://github.com/braktar).
 
 ### 1.5.2 (2021/02/06)
 

README.md

@@ -1228,6 +1228,7 @@ params do
   end
 end
 ```
+Note that, a `nil` value will call the custom coercion method, while a missing parameter will not.
 
 Example of use of `coerce_with` with a lambda (a class with a `parse` method could also have been used)
 It will parse a string and return an Array of Integers, matching the `Array[Integer]` `type`.

UPGRADING.md

@@ -1,6 +1,31 @@
 Upgrading Grape
 ===============
 
+
+### Upgrading to >= 1.5.3
+
+### Nil value and coercion
+
+A parameter with a value set to `nil` call custom coercion method, but the coercion is not triggered for missing parameters.
+
+```ruby
+class Api < Grape::API
+  params do
+    optional :value, type: Integer, coerce_with: ->(val) { val || 0 }
+  end
+
+  get 'example' do
+     params[:my_param]
+  end
+  get '/example', params: { value: nil }
+  # 1.5.2 = nil
+  # 1.5.3 = 0
+  get '/example', params: {}
+  # 1.5.2 = nil
+  # 1.5.3 = nil
+end
+```
+
 ### Upgrading to >= 1.5.1
 
 #### Dependent params