Added file.Close() call to file cache Get() method

The Get() method for the file cache was not closing files
that it opened. This will eventually cause an issue with
too many file descriptors being open.

closes go-spatial#586

cache/file/file.go

@@ -82,6 +82,7 @@ func (fc *Cache) Get(key *cache.Key) ([]byte, bool, error) {
 
 		return nil, false, err
 	}
+	defer f.Close()
 
 	val, err := ioutil.ReadAll(f)
 	if err != nil {