Objective-C: Fix memory leak in Patch (google#156)

Resolves google#156.

The NSMutableArray being assigned to Patch.diffs isn't being autoreleased correctly. Since it's being assigned to a synthesized property marked with retain, and the NSMutableArray is being initialized with alloc-init, the array needs to be autoreleased so that it doesn't leak.

See also Simperium/simperium-ios#89

Authored-by: Anthony Drendel <atdrendel@users.noreply.github.com>

objectivec/DiffMatchPatch.m

@@ -189,7 +189,7 @@ - (id)copyWithZone:(NSZone *)zone
 {
   Patch *newPatch = [[[self class] allocWithZone:zone] init];
 
-  newPatch.diffs = [[NSMutableArray alloc] initWithArray:self.diffs copyItems:YES];
+  newPatch.diffs = [[[NSMutableArray alloc] initWithArray:self.diffs copyItems:YES] autorelease];
   newPatch.start1 = self.start1;
   newPatch.start2 = self.start2;
   newPatch.length1 = self.length1;