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)