Update consumer-proguard-rules.pro (#2202)

See the following related issues:
- #704
- #806
- #2141
- aws-amplify/amplify-android#939

Author: raphkim

aws-android-sdk-core/consumer-proguard-rules.pro

@@ -2,6 +2,9 @@
 -keepnames class com.amazonaws.**
 -keepnames class com.amazon.**
 
+# Enums are not obfuscated correctly in combination with Gson
+-keepclassmembers enum * { *; }
+
 # Request handlers defined in request.handlers
 -keep class com.amazonaws.services.**.*Handler