Add a User-Agent when using the Lambda Runtime API (#98)

* Add a User-Agent when using the Lambda Runtime API

Author: bmoffatt

Diff for: Sources/AWSLambdaRuntimeCore/HTTPClient.swift

@@ -27,6 +27,8 @@ internal final class HTTPClient {
     private var state = State.disconnected
     private var executing = false
 
+    private static let headers = HTTPHeaders([("user-agent", "Swift-Lambda/Unknown")])
+
     init(eventLoop: EventLoop, configuration: Lambda.Configuration.RuntimeEngine) {
         self.eventLoop = eventLoop
         self.configuration = configuration
@@ -37,13 +39,15 @@ internal final class HTTPClient {
         self.execute(Request(targetHost: self.targetHost,
                              url: url,
                              method: .GET,
+                             headers: HTTPClient.headers,
                              timeout: timeout ?? self.configuration.requestTimeout))
     }
 
     func post(url: String, body: ByteBuffer?, timeout: TimeAmount? = nil) -> EventLoopFuture<Response> {
         self.execute(Request(targetHost: self.targetHost,
                              url: url,
                              method: .POST,
+                             headers: HTTPClient.headers,
                              body: body,
                              timeout: timeout ?? self.configuration.requestTimeout))
     }