Make project build pass with swift 4.0

- pod update
- Force compile rxcocoa and rxswift with swift 3.2

Pods/GCDWebServer/GCDWebServer/Requests/GCDWebServerURLEncodedFormRequest.m

@@ -31,16 +31,8 @@
 
 #import "GCDWebServerPrivate.h"
 
-@interface GCDWebServerURLEncodedFormRequest () {
-@private
-  NSDictionary* _arguments;
-}
-@end
-
 @implementation GCDWebServerURLEncodedFormRequest
 
-@synthesize arguments=_arguments;
-
 + (NSString*)mimeType {
   return @"application/x-www-form-urlencoded";
 }
@@ -49,12 +41,10 @@
   if (![super close:error]) {
     return NO;
   }
-  
+
   NSString* charset = GCDWebServerExtractHeaderValueParameter(self.contentType, @"charset");
   NSString* string = [[NSString alloc] initWithData:self.data encoding:GCDWebServerStringEncodingFromCharset(charset)];
   _arguments = GCDWebServerParseURLEncodedForm(string);
-  GWS_DCHECK(_arguments);
-  
   return YES;
 }