BUG X-RNCache header written to cache for redirects.

The X-RNCache header is not supposed to be written to the cache for redirects. This way, our system will get a chance to cache the redirected request. Unfortunately, I misunderstood how -[NSMutableURLRequest setAllHTTPHeaderFields:] works. I changed to use -[NSMutableURLRequest setValue:nil forHTTPHeaderField:@"X-RNCache"] instead.
Allow overriding -[RNCachingURLProtocol cachePathForRequest:]
Allow subclasses to determine when to cache.
Merge pull request #10 from hborders/extensible_cache_paths

Allow overriding -[RNCachingURLProtocol cachePathForRequest:]
Merge pull request #9 from hborders/bug_caching_header_present_in_cached_redirects

BUG X-RNCache header written to cache for redirects.
Merge remote-tracking branch 'hborders/extensible_caching'

Conflicts:
	RNCachingURLProtocol.h
Allow subclasses to determine when to cache.