[REST] add JSON support for /rest/headers/

doc/REST-interface.md

@@ -26,13 +26,11 @@ The HTTP request and response are both handled entirely in-memory, thus making m
 With the /notxdetails/ option JSON response will only contain the transaction hash instead of the complete transaction details. The option only affects the JSON response.
 
 ####Blockheaders
-`GET /rest/headers/<COUNT>/<BLOCK-HASH>.<bin|hex>`
+`GET /rest/headers/<COUNT>/<BLOCK-HASH>.<bin|hex|json>`
 
 Given a block hash,
 Returns <COUNT> amount of blockheaders in upward direction.
 
-JSON is not supported.
-
 ####Chaininfos
 `GET /rest/chaininfo.json`