Fix start() re-entrancy (#1) (#56)

Fixes an `EventSource.start()` re-entrancy problem

When:
1. `EventSource.start()` is called
2. `EventSource.start()` is called again, before the network response
from the first call has been received

Then:
- a second `URLSession` and `URLRequest` are created, and a new
`URLSessionDataTask` is started

The underlying issue is that `start()` is checking for `readyState` ==
`.raw`, but `readyState` isn't set until a network response has been
received.

Author: g-mark

Source/LDSwiftEventSource.swift

@@ -168,6 +168,7 @@ class EventSourceDelegate: NSObject, URLSessionDataDelegate {
                 self.logger.log(.info, "start() called on already-started EventSource object. Returning")
                 return
             }
+            self.readyState = .connecting
             self.urlSession = self.createSession()
             self.connect()
         }

Tests/LDSwiftEventSourceTests.swift

@@ -207,6 +207,17 @@ final class LDSwiftEventSourceTests: XCTestCase {
         XCTAssertEqual(handler.request.allHTTPHeaderFields?["X-LD-Header"], "def")
         es.stop()
     }
+    
+    func testStartRequestIsNotReentrant() {
+        var config = EventSource.Config(handler: mockHandler, url: URL(string: "http://example.com")!)
+        config.urlSessionConfiguration = sessionWithMockProtocol()
+        let es = EventSource(config: config)
+        es.start()
+        es.start()
+        _ = MockingProtocol.requested.expectEvent()
+        MockingProtocol.requested.expectNoEvent()
+        es.stop()
+    }
 
     func testSuccessfulResponseOpens() {
         var config = EventSource.Config(handler: mockHandler, url: URL(string: "http://example.com")!)