Don't let AFURLConnectionOperation accept a nil request

Closes #1064

AFNetworking/AFURLConnectionOperation.m

@@ -267,6 +267,8 @@ static BOOL AFSecKeyIsEqualToKey(SecKeyRef key1, SecKeyRef key2) {
 #endif
 
 - (id)initWithRequest:(NSURLRequest *)urlRequest {
+    NSParameterAssert(urlRequest);
+
     self = [super init];
     if (!self) {
 		return nil;

Tests/AFURLConnectionOperationTests.m

@@ -37,6 +37,10 @@
 
 #pragma mark -
 
+- (void)testInitWithNilRequestRaisesException {
+    expect(^{ (void)[[AFURLConnectionOperation alloc] initWithRequest:nil]; }).to.raiseAny();
+}
+
 - (void)testThatAFURLConnectionOperationInvokesWillSendRequestForAuthenticationChallengeBlock {
     NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"/path" relativeToURL:self.baseURL]];
     AFURLConnectionOperation *operation = [[AFURLConnectionOperation alloc] initWithRequest:request];