Rename userAgent -> userAgentString

Apparently Apple doesn't like it when asi-http synthesizes userAgent.

https://github.com/pokeb/asi-http-request/issues/309

@@ -193,7 +193,7 @@ typedef void (^ASIDataBlock)(NSData *data);
 	NSString *password;
 	
 	// User-Agent for this request
-	NSString *userAgent;
+	NSString *userAgentString;
 	
 	// Domain used for NTLM authentication
 	NSString *domain;
@@ -903,7 +903,7 @@ typedef void (^ASIDataBlock)(NSData *data);
 
 @property (retain) NSString *username;
 @property (retain) NSString *password;
-@property (retain) NSString *userAgent;
+@property (retain) NSString *userAgentString;
 @property (retain) NSString *domain;
 
 @property (retain) NSString *proxyUsername;

@@ -392,7 +392,7 @@ static NSOperationQueue *sharedQueue = nil;
 	[connectionInfo release];
 	[requestID release];
 	[dataDecompressor release];
-	[userAgent release];
+	[userAgentString release];
 
 	#if NS_BLOCKS_AVAILABLE
 	[self releaseBlocksOnMainThread];
@@ -1081,12 +1081,12 @@ static NSOperationQueue *sharedQueue = nil;
 	
 	// Build and set the user agent string if the request does not already have a custom user agent specified
 	if (![[self requestHeaders] objectForKey:@"User-Agent"]) {
-		NSString *userAgentString = [self userAgent];
-		if (!userAgentString) {
-			userAgentString = [ASIHTTPRequest defaultUserAgentString];
+		NSString *tempUserAgentString = [self userAgentString];
+		if (!tempUserAgentString) {
+			tempUserAgentString = [ASIHTTPRequest defaultUserAgentString];
 		}
-		if (userAgentString) {
-			[self addRequestHeader:@"User-Agent" value:userAgentString];
+		if (tempUserAgentString) {
+			[self addRequestHeader:@"User-Agent" value:tempUserAgentString];
 		}
 	}
 	
@@ -4991,7 +4991,7 @@ static NSOperationQueue *sharedQueue = nil;
 
 @synthesize username;
 @synthesize password;
-@synthesize userAgent;
+@synthesize userAgentString;
 @synthesize domain;
 @synthesize proxyUsername;
 @synthesize proxyPassword;