Fix CORSMiddleware configuration initializer parameter (vapor#2314)

* Makes sure CORSMiddleware always adds before ErrorMiddleware

* Deprecates the CORSMiddleware initializer which takes an [String]

* Added the doc comment

* Tanner's requested changes

* Switched to a 'convenience' initializer

Sources/Vapor/Deprecations/CORSMiddleware+Configuration+exposedHeaders.swift

@@ -0,0 +1,27 @@
+extension CORSMiddleware.Configuration {
+    /// Instantiate a CORSConfiguration struct that can be used to create a `CORSConfiguration`
+    /// middleware for adding support for CORS in your responses.
+    ///
+    /// - parameters:
+    ///   - allowedOrigin: Setting that controls which origin values are allowed.
+    ///   - allowedMethods: Methods that are allowed for a CORS request response.
+    ///   - allowedHeaders: Headers that are allowed in a response for CORS request.
+    ///   - allowCredentials: If cookies and other credentials will be sent in the response.
+    ///   - cacheExpiration: Optionally sets expiration of the cached pre-flight request in seconds.
+    ///   - exposedHeaders: Headers exposed in the response of pre-flight request.
+    @available(*, deprecated, message: "exposedHeaders parameter now accepts [HTTPHeaders.Name]")
+    public init(
+        allowedOrigin: CORSMiddleware.AllowOriginSetting,
+        allowedMethods: [HTTPMethod],
+        allowedHeaders: [HTTPHeaders.Name],
+        allowCredentials: Bool = false,
+        cacheExpiration: Int? = 600,
+        exposedHeaders: [String]
+    ) {
+        let exposed = exposedHeaders.map { HTTPHeaders.Name($0) }
+        self.init(
+            allowedOrigin: allowedOrigin, allowedMethods: allowedMethods, allowedHeaders: allowedHeaders,
+            allowCredentials: allowCredentials, cacheExpiration: cacheExpiration, exposedHeaders: exposed
+        )
+    }
+}

Sources/Vapor/Middleware/CORSMiddleware.swift

@@ -97,14 +97,14 @@ public final class CORSMiddleware: Middleware {
             allowedHeaders: [HTTPHeaders.Name],
             allowCredentials: Bool = false,
             cacheExpiration: Int? = 600,
-            exposedHeaders: [String]? = nil
+            exposedHeaders: [HTTPHeaders.Name]? = nil
         ) {
             self.allowedOrigin = allowedOrigin
             self.allowedMethods = allowedMethods.map({ "\($0)" }).joined(separator: ", ")
-            self.allowedHeaders = allowedHeaders.map({ $0.description }).joined(separator: ", ")
+            self.allowedHeaders = allowedHeaders.map({ String(describing: $0) }).joined(separator: ", ")
             self.allowCredentials = allowCredentials
             self.cacheExpiration = cacheExpiration
-            self.exposedHeaders = exposedHeaders?.joined(separator: ", ")
+            self.exposedHeaders = exposedHeaders?.map({ String(describing: $0) }).joined(separator: ", ")
         }
     }