Expose cacheKey function as public (#2014)

* Expose cacheKey function as public
* Remove @testable attribute to require public access to cacheKey function

Author: calvincestari

Sources/Apollo/GraphQLSelectionSet.swift

@@ -61,7 +61,7 @@ public struct GraphQLField: GraphQLSelection {
     self.type = type
   }
 
-  func cacheKey(with variables: [String: JSONEncodable]?) throws -> String {
+  public func cacheKey(with variables: [String: JSONEncodable]?) throws -> String {
     if
       let argumentValues = try arguments?.evaluate(with: variables),
       argumentValues.apollo.isNotEmpty {

Tests/ApolloTests/CacheKeyForFieldTests.swift

@@ -1,5 +1,5 @@
 import XCTest
-@testable import Apollo
+import Apollo
 import ApolloTestSupport
 import StarWarsAPI