Allow forcebuy price to be a string by converting it to float

fix #3970

freqtrade/rpc/api_server.py

@@ -508,6 +508,8 @@ class ApiServer(RPC):
         """
         asset = request.json.get("pair")
         price = request.json.get("price", None)
+        price = float(price) if price is not None else price
+
         trade = self._rpc_forcebuy(asset, price)
         if trade:
             return jsonify(trade.to_json())

freqtrade/rpc/rpc.py

@@ -524,7 +524,7 @@ class RPC:
         stake_currency = self._freqtrade.config.get('stake_currency')
         if not self._freqtrade.exchange.get_pair_quote_currency(pair) == stake_currency:
             raise RPCException(
-                f'Wrong pair selected. Please pairs with stake {stake_currency} pairs only')
+                f'Wrong pair selected. Only pairs with stake-currency {stake_currency} allowed.')
         # check if valid pair
 
         # check if pair already has an open pair