Fix issue with OHHTTPStubsResponse+JSON when nil headers dictionary

Author: AliSoftware

OHHTTPStubs.podspec

@@ -1,7 +1,7 @@
 Pod::Spec.new do |s|
 
   s.name         = "OHHTTPStubs"
-  s.version      = "2.3.0"
+  s.version      = "2.3.1"
 
   s.summary      = "Stubbing framework for network requests."
   s.description  = <<-DESC

OHHTTPStubs/OHHTTPStubsResponse+JSON.m

@@ -18,16 +18,14 @@ + (instancetype)responseWithJSONObject:(id)jsonObject
 {
     if (!httpHeaders[@"Content-Type"])
     {
-        // If httpHeaders is nil, we can't make a mutable copy.
-        NSMutableDictionary *mutableHeaders = httpHeaders ? [httpHeaders mutableCopy] : [NSMutableDictionary dictionary];
+        NSMutableDictionary* mutableHeaders = [NSMutableDictionary dictionaryWithDictionary:httpHeaders];
         mutableHeaders[@"Content-Type"] = @"application/json";
-        httpHeaders = [mutableHeaders copy]; // make immutable again
+        httpHeaders = [NSDictionary dictionaryWithDictionary:mutableHeaders]; // make immutable again
     }
 
     return [self responseWithData:[NSJSONSerialization dataWithJSONObject:jsonObject options:0 error:nil]
                        statusCode:statusCode
-                          headers:httpHeaders
-            ];
+                          headers:httpHeaders];
 }
 
 @end