middleware and headers docs

3.0/docs/http/middleware.md

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+# Middleware
+
+Middleware are a step in Vapor's responder chain. They're capable of modifying Requests/Responses, preventing the chain from continuing and transforming the data flow.
+
+They can be employed for authorization checks, logging and a wide range of other functionalities.
+
+## Implementing a Middleware
+
+The following example is a middleware that will prevent all [requests](request.md) from going to their respective [responder](responder.md) unless the origin has a special header set. In the case of a missing header, [status code](status.md) 404 (not found) will be returned.
+
+Don't secure your APIs using this example code, it's very unsafe and exclusively to be used as a test.
+
+```swift
+public final class SpecialHeaderCheckMiddleware: Middleware {
+  public func respond(to request: Request, chainingTo next: Responder) throws -> Future<Response> {
+    guard request.headers["Secret-Header"] == "MagicK3y" else {
+      return Response(status: .notFound)
+    }
+
+    return try next.respond(to: request)
+  }
+}
+```
+
+## Intercepting/transforming Responses
+
+The following example demonstrates a middleware that creates a session token for new users.
+
+```swift
+public final class SessionTokenMiddleware: Middleware {
+  func generateSessionToken() throws -> String {
+    // Generate token here ...
+    return token
+  }
+
+  public func respond(to request: Request, chainingTo next: Responder) throws -> Future<Response> {
+    let response = try next.respond(to: request)
+
+    guard request.cookies["session"] != nil else {
+      response.cookies["session"] = try generateSessionToken()
+
+      return response
+    }
+
+    return response
+  }
+}
+```

3.0/docs/web/headers.md

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+# Headers
+
+HTTP Headers are the metadata of a request/response. They can provide a wide variety of information.
+
+`Headers` are an array of key-value pairs. As such it's possible, but not common for multiple pairs to have the same key.
+
+## Creating a Headers object
+
+The most common syntax for creating Headers is a dictionary literal.
+
+```swift
+let headers: Headers = [
+  .contentType: "text/html"
+]
+```
+
+The left hand side (key) is a `Header.Name`.
+
+A name can also be initialized with a String literal.
+
+```swift
+let headers: Headers = [
+  "Content-Type": "text/html"
+]
+```
+
+## Accessing headers
+
+There are two ways to access Headers. Either by accessing a single (the first) value, or all values.
+
+### A single value example:
+
+```swift
+let headers: Headers = [
+  .contentType: "text/html"
+]
+
+print(headers[.contentType]) // prints "text/html"
+```
+
+### Accessing all values example:
+
+```swift
+let headers: Headers = [
+  .setCookie: "session=afasfwrw3qr241j4qwmdsijfo13k43",
+  .setCookie: "awesome=true"
+]
+
+// prints ["session=afasfwrw3qr241j4qwmdsijfo13k43", "awesome=true"]
+print(headers[valuesFor: .contentType])
+```