Avoid coercing parameter with multiple types to an empty Array

If an optional parameter has multiple types (e.g. [File, String]) and a
nil parameter is passed, Grape would previously coerce the value to an
empty Array. This can break certain APIs that treat nil values
differently from an empty Array.

Closes ruby-grape#2018

lib/grape/validations/validators/coerce.rb

@@ -68,9 +68,10 @@ def coerce_value(val)
         # define default values for structures, the dry-types lib which is used
         # for coercion doesn't accept nil as a value, so it would fail
         if val.nil?
-          return []      if type == Array || type.is_a?(Array)
+          return []      if type == Array
           return Set.new if type == Set
           return {}      if type == Hash
+          return val
         end
 
         converter.call(val)

spec/grape/validations/validators/coerce_spec.rb

@@ -647,6 +647,19 @@ def self.parsed?(value)
         expect(last_response.body).to eq('String')
       end
 
+      it 'respects nil values' do
+        subject.params do
+          optional :a, types: [File, String]
+        end
+        subject.get '/' do
+          params[:a].class.to_s
+        end
+
+        get '/', a: nil
+        expect(last_response.status).to eq(200)
+        expect(last_response.body).to eq('NilClass')
+      end
+
       it 'fails when no coercion is possible' do
         subject.params do
           requires :a, types: [Boolean, Integer]