DISCUSSION: No way to stub server response which will lack a Content-Length header

This really isn't a bug, it's more of how OHTTPSTubs is currently designed.
The OHHTTPStubsResponse object will automatically add a Content-Length header if you didn't specify one in the OHHTTPStubsResponse.

```
return [OHHTTPStubsResponse responseWithFileAtPath:OHPathForFile(@"Sample-Video.mp4", [weakSelf class])
statusCode:200
headers:@{

@"Server":@"openresty/1.7.4.1",
@"ETag" :@"3c8f0eb7eae10fd2b42eb41d4e0568e7:1459461318",
@"Last-Modified" : @"Sat, 27 Apr 2013 00:44:54 GMT",
@"Content-Type" : @"video/mp4",
// @"Content-Length" : 12345,
@"Expires" : @"Tue, 1 Jan 2036 18:43:49 GMT",
@"Vary" : @"Accept-Encoding",
@"Date" : @"Fri, 24 Jun 2016 20:54:02 GMT"
}];
```
@AliSoftware, what do you think about if we can set the Content-Length to [NSNull null] and check for that in the OHHTTPStubsResponse object upon its creation, to conditionally ignore the Content-Length header? We actually have a case that we encountered where a server MAY fail to send the Content-Length header on some multi-part stuff.
I'd like a way to test this. Easy enough to do. Are you open to a PR that allows this?