post request indexing improvements

[ikreymer]

Improvements for POST indexing:

• store post (and other method) body in requestBody as query string
• index application/json, and try text/plain as json, to convert primitive values to query args, eg {"a": "b", "c": "d"} -> a=b&c=d
• store method in method field, add to urlkey as __wb_method=...
• implementation in sync with warcio.js