Moved the clearAuthorizationHeader call

This allows authorization to be set, and used, outside the call to
`authenticateUsingOAuthWithPath...`

Author: brendanjerwin

AFOAuth2Client/AFOAuth2Client.m

@@ -168,8 +168,6 @@ - (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"];
 
@@ -199,6 +197,7 @@ - (void)authenticateUsingOAuthWithPath:(NSString *)path
 
         [credential setRefreshToken:refreshToken expiration:expireDate];
 
+        [self clearAuthorizationHeader];
         [self setAuthorizationHeaderWithCredential:credential];
 
         if (success) {