Suppressed GNU expression and Enum assignment warnings from clang

After enabling as many warnings as I found reasonable (my current OTHER_CFLAGS is -Wall -Wextra -Weverything -Wno-objc-missing-property-synthesis -Wno-direct-ivar-access -Wno-documentation -Wno-unused-parameter -Wno-arc-repeated-use-of-weak -Wno-implicit-retain-self) I worked on suppressing those that I found irrelevant. Here's a patch that let's AFNetworking work for more pedantic compiler settings.