migrate from oauth1 to oauth2 error 400

Got error 400 "Required parameter is missing: grant_type" when trying to 
migrate from Oauth1 to Oauth2. The request I sent was:
POST https://www.googleapis.com/oauth2/v3/token
Headers: 'authorization' => 'OAuth realm="", oauth_consumer_key="xxxxxx", 
oauth_nonce="fc293eca4049a932961f", 
oauth_signature="ibocvM5mJE7RtuPXFOmWZN7pAu8%3D", 
oauth_signature_method="HMAC-SHA1", oauth_timestamp="1427408997", 
oauth_token="1%2FcSvCYNIJHNJETF9gWEFWpDY1cJyDw1amHqNfqPeCCAI", 
oauth_version="1.0"'
content: 
grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Amigration%3Aoauth1&client_id=581780486572-ndod9e3j4d6fdphs30ksk6qm1ild47eu.apps.googleusercontent.com&client_secret=dfw2Vdfsl5FGnvIiqJEsJgg6

I've tried different things, changing the header, payload, but still 
doesn't work. Please help!!!!

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