Add ruby-grape#1956 to changelog

Author: ghiculescu

CHANGELOG.md

@@ -63,6 +63,7 @@
 * [#1971](https://github.com/ruby-grape/grape/pull/1971): Fix BigDecimal coercion - [@FlickStuart](https://github.com/FlickStuart).
 * [#1968](https://github.com/ruby-grape/grape/pull/1968): Fix args forwarding in Grape::Middleware::Stack#merge_with for ruby 2.7.0 - [@dm1try](https://github.com/dm1try).
 * [#1988](https://github.com/ruby-grape/grape/pull/1988): Refactored the full_messages method and stop overriding full_message - [@hosseintoussi](https://github.com/hosseintoussi).
+* [#1956](https://github.com/ruby-grape/grape/pull/1956): Make grape compliant with Rack spec. (Amongst other things, fixes the `undefined method [] for nil:NilClass` error when upgrading Rack.) - [@ioquatix](https://github.com/ioquatix)
 
 ### 1.3.0 (2020/01/11)