Refactor #generate_api_method to improve performance (#1726)

Author: jkowens

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#1722](https://github.com/ruby-grape/grape/pull/1722): Fix catch-all hiding multiple versions of an endpoint after the first definition - [@zherr](https://github.com/zherr).
 * [#1724](https://github.com/ruby-grape/grape/pull/1724): Optional nested array validation - [@ericproulx](https://github.com/ericproulx).
 * [#1725](https://github.com/ruby-grape/grape/pull/1725): Fix `rescue_from :all` documentation - [@Jelkster](https://github.com/Jelkster).
+* [#1726](https://github.com/ruby-grape/grape/pull/1726): Improved startup performance during API method generation - [@jkowens](https://github.com/jkowens).
 * Your contribution here.
 
 ### 1.0.1 (9/8/2017)

lib/grape/endpoint.rb

@@ -44,7 +44,7 @@ def run_before_each(endpoint)
       # @return [Proc]
       # @raise [NameError] an instance method with the same name already exists
       def generate_api_method(method_name, &block)
-        if instance_methods.include?(method_name.to_sym) || instance_methods.include?(method_name.to_s)
+        if method_defined?(method_name)
           raise NameError.new("method #{method_name.inspect} already exists and cannot be used as an unbound method name")
         end