Merge pull request AFNetworking#33 from tnwinc/master

Allow `setAuthorizationHeaderWithUsername` to work when called before getting OAuth credentials.

Author: mattt

AFOAuth2Client/AFOAuth2Client.m

@@ -167,8 +167,6 @@ - (void)authenticateUsingOAuthWithPath:(NSString *)path
     [mutableParameters setValue:self.secret forKey:@"client_secret"];
     parameters = [NSDictionary dictionaryWithDictionary:mutableParameters];
 
-    [self clearAuthorizationHeader];
-
     NSMutableURLRequest *mutableRequest = [self requestWithMethod:@"POST" path:path parameters:parameters];
     [mutableRequest setValue:@"application/json" forHTTPHeaderField:@"Accept"];
 
@@ -198,6 +196,7 @@ - (void)authenticateUsingOAuthWithPath:(NSString *)path
 
         [credential setRefreshToken:refreshToken expiration:expireDate];
 
+        [self clearAuthorizationHeader];
         [self setAuthorizationHeaderWithCredential:credential];
 
         if (success) {