Enable strict mode for Steep type checking

Fix @encoder class instance variable declaration to use self.@encoder so
Steep's strict diagnostics can resolve it on the singleton class.

Author: sferik

Steepfile

@@ -4,6 +4,8 @@ target :lib do
   signature "sig"
   check "lib"
 
+  configure_code_diagnostics(Steep::Diagnostic::Ruby.strict)
+
   library "pathname"
   library "securerandom"
   library "stringio"

sig/http/form_data/urlencoded.rbs

@@ -3,7 +3,7 @@ module HTTP
     class Urlencoded
       include Readable
 
-      @encoder: _Encoder?
+      self.@encoder: _Encoder?
 
       # Sets custom form data encoder implementation
       def self.encoder=: (untyped implementation) -> void