Add OAuth Echo methods to TWRequest

TWRequest+OAuthEcho.h

#import <Twitter/Twitter.h>
#import <Accounts/Accounts.h>


@interface TWRequest (OAuthEcho)

/*
 * code example:
 *
 * NSURL *url = [NSURL URLWithString:@"https://api.twitter.com/1/account/verify_credentials.json"];
 * TWRequest *request = [[TWRequest alloc] initWithURL:url parameters:nil requestMethod:TWRequestMethodGET];
 * [request setAccount:account];
 * NSDictionary *headerFields = [request oauthEchoHeaderFields];
 *
 * // NSMutableURL
 * NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:apiURL];
 * [headerFields enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
 *   [request setValue:obj forHTTPHeaderField:key];
 * }];
 *
 * // MKNetWorkKit
 * MKNetworkEngine *engine = [[[MKNetworkEngine alloc] initWithHostName:@"example.com" customHeaderFields:headerFields];
 *
 * see:
 *   https://gist.github.com/1575035
 */
- (NSDictionary *)oauthEchoHeaderFields;

@end

TWRequest+OAuthEcho.m

#import "TWRequest+OAuthEcho.h"

@implementation TWRequest (OAuthEcho)

- (NSDictionary *)oauthEchoHeaderFields
{
    NSURLRequest *signedURLRequest = [self signedURLRequest];
    
    NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
    [dictionary setValue:[[signedURLRequest URL] absoluteString]
                  forKey:@"X-Auth-Service-Provider"];
    [dictionary setValue:[signedURLRequest valueForHTTPHeaderField:@"Authorization"]
                  forKey:@"X-Verify-Credentials-Authorization"];
    
    return [NSDictionary dictionaryWithDictionary:dictionary];
}

@end