Fix return type restriction for ENV.fetch (#14919)

spec/std/env_spec.cr

@@ -137,6 +137,10 @@ describe "ENV" do
         ENV.fetch("2")
       end
     end
+
+    it "fetches arbitrary default value" do
+      ENV.fetch("nonexistent", true).should be_true
+    end
   end
 
   it "handles unicode" do

src/env.cr

@@ -60,7 +60,7 @@ module ENV
 
   # Retrieves a value corresponding to the given *key*. Return the second argument's value
   # if the *key* does not exist.
-  def self.fetch(key, default) : String?
+  def self.fetch(key, default : T) : String | T forall T
     fetch(key) { default }
   end