Improve typehints

Author: barryvdh

src/Omnipay/Common/Currency.php

@@ -62,7 +62,7 @@ public function getDecimals()
      * Find a specific currency
      *
      * @param  string $code The three letter currency code
-     * @return mixed  A Currency object, or null if no currency was found
+     * @return static|null  A Currency object, or null if no currency was found
      */
     public static function find($code)
     {

src/Omnipay/Common/Message/AbstractRequest.php

@@ -392,7 +392,7 @@ public function setCurrency($value)
     /**
      * Get the payment currency number.
      *
-     * @return integer
+     * @return string|null
      */
     public function getCurrencyNumeric()
     {
@@ -423,6 +423,7 @@ private function getCurrencyDecimalFactor()
     /**
      * Format an amount for the payment currency.
      *
+     * @param double $amount
      * @return string
      */
     public function formatCurrency($amount)